Every year the same surprise returns: Christmas always falls on December 25, but Easter moves. Sometimes it comes in late March, sometimes in late April, and the greatest Christian feast seems to slide from one year to the next without any obvious logic. Many people imagine that the Church chooses the date at random, or that it follows an old lunar superstition. The reality is quite different: Easter obeys a precise rule, inherited at once from the Bible, the Jewish calendar and a patient work of Christian astronomy that stretches across more than fifteen hundred years.
If Easter changes date, it is not through negligence: it is because it remains faithful to its origin. The Resurrection of Christ took place on "the first day of the week" (Mark 16:2), after a Sabbath, during the Jewish Passover - a feast itself fixed not by the sun but by the moon[1]. By keeping a lunar reference point, the Church preserves the historical bond between the Resurrection of Christ and the Passover in which it was accomplished. To understand why Easter moves, then, is to return to the night when "Christ, our Passover, has been sacrificed" (1 Corinthians 5:7)[2].
The Rule: a Sunday Governed by the Moon and the Equinox
The exact statement
The date of Easter has been fixed, since Christian antiquity, by a formula that became classical:
"Easter-day (...) is always the first Sunday after the Full Moon which happens upon, or next after the Twenty-first Day of March; and if the Full Moon happens upon a Sunday, Easter-day is the Sunday after." — Formulation of the ecclesiastical computus, transmitted by Dionysius Exiguus and Bede then formalized by Clavius; taken up in England by the Calendar (New Style) Act of 1750 and the Book of Common Prayer[3]
Three elements therefore govern the date: a reference equinox (March 21), a full moon, and a Sunday. Easter is the first Sunday that follows the first full moon occurring on or after March 21. Since the moon and the weekday never synchronize exactly with the solar year, the combination of the three makes the feast move within a fixed window.
From March 22 to April 25: the window of possible dates
The result is that Easter can never fall before March 22 or after April 25 (in the Gregorian calendar)[4]. The earliest possible date, March 22, requires the paschal full moon to fall on Saturday, March 21, the equinox: Easter is then the next day. The latest possible date, April 25, is the opposite case, when the full moon narrowly misses the equinox and pushes everything back by a whole lunar month. Between these two limits, the feast can occupy thirty-five different dates.
These extremes are rare. Easter has not fallen on March 22 since 1818, and will not do so again until 2285. April 25 last occurred in 1943 and will return in 2038[5]. In most years, Easter falls quietly between late March and mid-April.
An "ecclesiastical" moon and equinox, not astronomical ones
The point most often overlooked is crucial: the full moon and equinox used for Easter are not the actual astronomical phenomena, but calendar conventions. The equinox is fixed once and for all on March 21, even though the astronomical equinox most often falls on March 20[6]. Likewise, the "paschal full moon" is an ecclesiastical moon: the fourteenth day of a lunar month reconstructed by calculation, which may differ by one or two days from the true full moon observed in the sky[7]. This convention is not a crude approximation: it allows the date to be fixed years in advance, everywhere in the world, without depending on variable local observation. The Church preferred a universal and stable rule to uncertain observation.
A Jewish Feast First: from Passover to the Passover of Christ
The word "Easter": Pessah, Pascha, and no pagan root
A persistent legend claims that Easter is a recovered pagan feast named after a spring goddess. This is false for the French word Paques and for the Christian term Pascha. Paques comes directly from the Latin Pascha, itself from the Greek Paskha, a transcription of the Hebrew פֶּסַח (Pessah), the Jewish Passover[8]. The Hebrew root pasaḥ means "to pass over, to spare": it evokes the destroying angel "passing over" the Hebrew houses marked with the blood of the lamb on the night of the Exodus (Exodus 12:13)[9]. The very name of the Christian feast therefore proclaims its biblical rootedness: Easter is the Passover fulfilled.
The confusion with a goddess comes only from the English word "Easter" (and the German Ostern). In the eighth century, Bede the Venerable notes that the Anglo-Saxons called the month of April Eosturmonath, after a goddess named Eostre:
"Eosturmonath, which is now interpreted as the paschal month, once took its name from a goddess of theirs called Eostre, in whose honour feasts were celebrated in that month." — Bede the Venerable, De temporum ratione, ch. 15[10]
But this remark concerns only the Germanic name of the feast, transferred to the season in which the Resurrection was celebrated. It says nothing about the feast itself, nor about the word Pascha, which has no pagan origin. Specialists also note that Bede may have inferred the existence of the goddess from the name of the month alone[11].
The Resurrection at the heart of Passover
The Gospels place the Passion and Resurrection of Christ during the Jewish feast of Passover, in Jerusalem[12]. Jesus dies at the time when the paschal lambs were being sacrificed, and rises on "the first day of the week", that is, Sunday. Saint Paul draws a whole theology from this convergence:
"Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover, has been sacrificed. Let us therefore celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth." — Saint Paul, 1 Corinthians 5:7-8[13]
That is why the Christian date remains tied to the Jewish date: it keeps the memory of the precise moment of salvation. Since the Jewish Passover is lunar - fixed on the 14th day of the month of Nisan, the first month of spring - the Christian Passover will inherit this lunar character, while moving it to Sunday, the day of the Resurrection.
The Great Dispute of the First Centuries: When Should Easter Be Celebrated?
The Quartodecimans: Passover on 14 Nisan
As early as the second century, Christians did not agree on the day. In Asia Minor, many celebrated Pascha on the 14th day of Nisan, exactly like the Jewish Passover, whatever the day of the week: they were called the Quartodecimans ("partisans of the fourteenth")[14]. Elsewhere, especially in Rome and Alexandria, Christians always waited for the following Sunday, so that the feast of the Resurrection would coincide with the day of the Resurrection.
Polycarp and Anicetus: disagreement in peace
Around 154-155, the aged bishop Polycarp of Smyrna, disciple of the apostle John, came to Rome to meet Pope Anicetus. The two men could not reach agreement on the date, but - a beautiful detail reported by Saint Irenaeus - they did not break communion:
"Anicetus could not persuade Polycarp not to observe what he had always observed with John (...). Yet they remained in communion with one another, and in the church Anicetus yielded the Eucharist to Polycarp, out of respect. And they parted in peace." — Saint Irenaeus, quoted by Eusebius of Caesarea, Ecclesiastical History, V, 24[15]
Victor I: the threat of excommunication
A generation later, around 190-195, Pope Victor I wanted to impose Roman usage and threatened to excommunicate the Churches of Asia that remained Quartodeciman[16]. This time the tone hardened. But several bishops, including Saint Irenaeus of Lyons - though himself favourable to the Sunday usage - reproached the pope for his haste and pleaded for patience and unity. Irenaeus recalled precisely the example of Polycarp and Anicetus: Christians can differ over a date without breaking communion[17]. The crisis was not resolved by force, but it revealed an urgent need: a common rule for the whole Church.
The Council of Nicaea (325): Unity, Not the Formula
What Nicaea really decided
It is often said that the Council of Nicaea "decreed" the famous formula "the first Sunday after the full moon after the equinox". That is an inaccurate simplification. The twenty surviving canons of Nicaea contain no canon on Easter[18]. What the council actually established, through the letter of Emperor Constantine and the synodal letter, can be summarized in three points:
- All Christians must celebrate Easter on the same day, everywhere.
- The feast must fall on a Sunday.
- The calculation must be independent of the Jewish calendar, so as not to depend on a computus external to the Church.
Constantine's letter to the Churches after the council is explicit about the goal of unity:
"This most holy feast should be celebrated by all on one and the same day. (...) It appeared unworthy that, in the celebration of this most holy solemnity, we should follow the custom of the Jews (...). Let us therefore all be of one accord." — Emperor Constantine, letter after the Council of Nicaea, quoted by Eusebius, Life of Constantine, III, 18-19[19]
The formula and Alexandria: later developments
The precise arithmetical formula (equinox on March 21, full moon, following Sunday) and the role of Alexandria as a centre of calculation do not appear in the Nicene texts: they are later developments. In the wake of the council, the Church of Alexandria, with its long astronomical tradition, in practice calculated the date each year and announced it to the rest of Christendom, while Rome ratified it[20]. The paschal tables of Bishop Cyril of Alexandria, covering 95 years (from 437 to 531), fixed the Alexandrian computus that would prevail in the West. Nicaea, then, did not deliver an equation: it laid down a principle of unity that later centuries translated into calculation.
How the Church Calculates Easter: the Computus
The problem: coordinating Sun and Moon
The whole difficulty comes from the fact that the solar year (365.2422 days) and the lunar month (29.53 days) never fit exactly into one another. Twelve lunations make about 354 days - eleven fewer than a solar year. Year after year, the phases of the moon therefore shift by eleven days in relation to the civil calendar. To calculate Easter is to solve this disagreement: this is the computus, the art of calculating the date of the movable feasts, which for centuries stood at the summit of medieval science[21].
The Metonic cycle: nineteen years to bring things back into order
The key was discovered by the Greek astronomer Meton of Athens, around 432 BC: after nineteen solar years, the phases of the moon fall again almost exactly on the same calendar dates[22]. Nineteen solar years are indeed approximately equal to 235 lunar months. This Metonic cycle of nineteen years became the backbone of the whole Christian computus: once one knows where the year stands in the cycle, one can recover the position of the moon.
Golden number and epact
Two tools translate this cycle into numbers. The golden number indicates the rank of the year in the nineteen-year cycle (a number from 1 to 19)[23]. The epact gives the age of the moon - the number of days elapsed since the last new moon - on a fixed date in the year[24]. With these two numbers, the computus can recover the date of the paschal full moon without observing the sky. The table below summarizes the vocabulary of the computus:
| Term | Definition | Role in calculating Easter |
|---|---|---|
| Metonic cycle | 19 solar years ≈ 235 lunar months | Backbone: the moon returns to the same dates |
| Golden number | Rank of the year in the cycle (1 to 19) | Locates the year within the lunar cycle |
| Epact | Age of the moon on a fixed date | Locates new and full moons |
| Ecclesiastical moon | Lunar month reconstructed by calculation | Gives the "paschal full moon" (14th day) |
| Ecclesiastical equinox | Fixed conventional date: March 21 | Starting point of the search |
The Metonic cycle is not perfect: it accumulates about one day of error every two and a half centuries[25]. This slight drift, negligible over a few generations, would eventually become a serious problem over the centuries - and would require reform.
From Paschal Tables to Latin Christendom
Victorius of Aquitaine and Dionysius Exiguus
To spread the computus, Christians drew up paschal tables giving the date of Easter year by year. In 457, at the request of the Roman archdeacon Hilary (the future pope), Victorius of Aquitaine established a great 532-year cycle[26]. But his tables sometimes gave two competing dates, Latin and Greek. It was the monk Dionysius Exiguus who, in 525, at the request of Pope John I, decided in favour of the Alexandrian computus by extending Cyril's tables[27]. In doing so, Dionysius rendered an immense service to all civilization: to number his years, he refused to count from the persecuting emperor Diocletian and chose to begin from the Incarnation of Christ. Thus he invented the Christian era, the "Anno Domini" ("year of the Lord") system still used throughout the world[28].
The Synod of Whitby (664) and Bede the Venerable
Unity was not immediate everywhere. In the British Isles, Celtic Christians followed an older computus (an 84-year cycle), different from the Roman one. The disagreement became striking at the Northumbrian court, where King Oswiu, of Celtic tradition, celebrated Easter while the queen, of Roman tradition, was still fasting. The Synod of Whitby, in 664, decided in favour of Rome[29]. The monk Wilfrid defended Roman usage there by invoking the authority received by Saint Peter:
"You are Peter, and on this rock I will build my Church (...) ; I will give you the keys of the kingdom of heaven." — Gospel according to Saint Matthew, 16:18-19, the decisive argument at Whitby[30]
Half a century later, the English monk Bede the Venerable systematized the whole science of the computus in his great treatise De temporum ratione ("The Reckoning of Time", 725), which remained a reference manual throughout the Middle Ages[31]. It was also Bede who, through his Ecclesiastical History (around 731), spread Dionysius Exiguus's Anno Domini system throughout the West.
The Gregorian Reform of 1582: Setting the Date Right Again
The problem: a drifting equinox
The Julian calendar, in use since Julius Caesar, counted a year of 365.25 days - about eleven minutes too long each year. Across centuries, those minutes accumulated: by the sixteenth century, the astronomical spring equinox fell around March 11, while the Easter computus still supposed it to be on March 21. The discrepancy had reached ten days. Easter was therefore being celebrated farther and farther from its real season, and the error threatened to throw everything out of order[32].
Inter gravissimas: ten days removed
Pope Gregory XIII promulgated the reform by the bull Inter gravissimas on February 24, 1582[33]. To recover the accumulated delay, ten days were removed at once: in 1582, Thursday, October 4 was immediately followed by Friday, October 15[34]. The weekly cycle itself was not interrupted: Thursday was indeed followed by Friday. Only the numbered calendar dates jumped. The equinox was thus brought back to March 21, as in the time of Nicaea.
A clock regulated for centuries
To prevent the error from returning, the reform changed the rule for century leap years: a year divisible by 100 is no longer a leap year, unless it is also divisible by 400. Thus 1700, 1800 and 1900 were common years, but 1600 and 2000 remained leap years[35]. The lunar tables were also corrected to make up for the drift of the Metonic cycle.
The author of the system was the Calabrian physician and astronomer Aloysius Lilius (Luigi Lilio), who died in 1576, six years before the reform was adopted; his brother Antonio presented the manuscript to the pope[36]. The Jesuit Christophorus Clavius, a renowned mathematician, elaborated, refined and publicly defended the calculation against its critics[37]. The Gregorian calendar, designed first to fix the date of Easter correctly, became the civil calendar of almost the whole planet.
Why the Orthodox Do Not Celebrate Easter on the Same Day
The same rule, two calendars
Catholics and Orthodox apply exactly the same Nicene rule: the first Sunday after the first full moon after the spring equinox. Yet their dates often differ - sometimes by one week, sometimes by several. The reason is not doctrinal but calendrical: the Orthodox Churches calculate Easter according to the unreformed Julian calendar, while the West uses the Gregorian calendar[38].
Thirteen days of difference, and divergent lunar tables
Two discrepancies accumulate. First, Julian "March 21" today corresponds to April 3 Gregorian: the Orthodox reference equinox is therefore thirteen days behind the astronomical equinox[39]. Second, the lunar tables of the two computuses have diverged since 1582: the ecclesiastical full moon is not calculated in the same way. When these discrepancies are combined, Orthodox Pascha falls on the same day as Latin Easter, or one week later, or four, even five weeks later - the maximum gap being thirty-five days[40].
The legend of "always after the Jewish Passover"
It is often said that the Orthodox have an additional rule: to celebrate Easter always after the Jewish Passover. In reality, historians and canonists see this as a misunderstanding, not an authentic canon. Nicaea's concern was not to depend on the Jewish calculation, not to require Easter to fall after Jewish Passover. The fact that, under the Julian computus, Orthodox Pascha almost always falls after Jewish Passover is a consequence of the calendar discrepancy, which, according to several historians, the Byzantine canonist John Zonaras (twelfth century) wrongly turned into a rule[41]. The distinction matters: this is a calendar effect, not a theological principle.
When the dates coincide
In some years, the two computuses agree. This happened in 2025, when Catholics and Orthodox celebrated Easter together on April 20[42]. Such coincidences occur several times each decade; the next will be in 2028, 2031 and 2034[43]. They remind us that the division concerns a calculation, not faith in the Resurrection.
Toward a Common Date for Easter?
What the Church is ready to accept
Far from treating the computus as a dogma, the Catholic Church has declared itself open to a common date, even a fixed one, provided it is done in unity. The Second Vatican Council, in an appendix to the constitution Sacrosanctum Concilium (1963), stated this explicitly:
"The Sacred Council would not object if the feast of Easter were assigned to a particular Sunday of the Gregorian Calendar, provided that those whom it may concern, especially the brethren who are not in communion with the Apostolic See, give their assent." — Second Vatican Council, Sacrosanctum Concilium, Appendix[44]
Aleppo 1997 and recent appeals
In 1997, an ecumenical consultation held in Aleppo, Syria, by the World Council of Churches proposed an elegant solution: keep the Nicene rule, but calculate the equinox and full moon according to real astronomical data, observed from the meridian of Jerusalem[45]. The proposal, planned for implementation in 2001, was never adopted for lack of general agreement. Recent popes have nevertheless renewed the appeal. Pope Francis, more than once, judged the division of Christians over the date of the Resurrection scandalous and said he was ready to renounce the Latin computus for a common date[46]. The coincidence of April 20, 2025, which fell in the year of the 1700th anniversary of the Council of Nicaea, was welcomed as a providential sign and an opportunity to move toward unity[47].
The Spiritual Meaning: Why a Movable Date?
Fidelity, not chance
One might think a fixed date would be simpler. But Easter's mobility is not a defect: it is a fidelity. By keeping the moon and spring, the Church keeps alive the link between the Resurrection and the Jewish Passover in which it was fulfilled, between the New Covenant and the Old. Easter dances to the rhythm of the moon like the Passover of Moses, because Christ did not abolish the Law but fulfilled it (Matthew 5:17)[48].
The springtime of Redemption
Nor is the date indifferent in its symbolism. Easter always falls in spring, when night recedes and life reappears, after the great fast of Lent. The Catechism recalls that Sunday, the "first day of the week", memorial of Creation and Resurrection, is "the day par excellence of the liturgical assembly" and the heart of the whole Christian year[49]. Every movable Easter, every year on its new date, repeats the same unchanging truth, sung by the Exsultet, the proclamation of the Easter night, celebrating the night when Christ, breaking the bonds of death, rose victorious from the underworld[50].
Whether Easter falls on March 22 or April 25, the Resurrection itself does not change. The date moves; the mystery remains. And it is in order to serve this mystery better - to remain faithful to the night when "Christ, our Passover, has been sacrificed" - that the Church has accepted, for seventeen centuries, to celebrate her greatest joy on a date that moves.
Further Reading
- Why does Lent last forty days? - the forty days of preparation that lead to Easter.
- Why is Christmas celebrated on December 25? - the other great date of the Christian calendar, fixed this time.
- Why is the Resurrection of Jesus essential? - the very heart of the feast of Easter.
- Why did Jesus have to die? - the Passion that Easter completes in glory.
- Why is Pentecost important? - the movable feast that crowns Eastertide.
- The Council of Nicaea (325) - the council that wanted one date of Easter for the whole Church.
- What does the Church say about Judaism? - the link between the Jewish Passover and the Christian Passover.
Notes and References
The Jewish Passover (Pessah) is fixed on the 14th day of Nisan, the first month of spring in the Hebrew lunisolar calendar (Exodus 12:1-6; Leviticus 23:5). Its date therefore depends on the moon, not the sun. Bible de Jerusalem, Cerf, Paris, 1998.
1Saint Paul, 1 Corinthians 5:7: "Pascha nostrum immolatus est Christus" ("Christ, our Passover, has been sacrificed"). Bible de Jerusalem, Cerf, Paris, 1998.
2The computus comes from Dionysius Exiguus (525 paschal table extending Cyril of Alexandria), systematized by Bede the Venerable (De temporum ratione, 725) and formalized for the Gregorian calendar by Christophorus Clavius (Explicatio, 1603). The most quoted English legal formulation appears in the Calendar (New Style) Act 1750 (the British law adopting the Gregorian calendar) and the Book of Common Prayer: "Easter-day (...) is always the first Sunday after the Full Moon which happens upon, or next after the Twenty-first Day of March; and if the Full Moon happens upon a Sunday, Easter-day is the Sunday after." — an Anglican restatement, not the founding authority.
3In the Gregorian calendar, Easter cannot fall before March 22 or after April 25: thirty-five possible dates. United States Naval Observatory, "The Date of Easter": "Easter can never occur before March 22 or later than April 25."
4Easter on March 22 (earliest date): 1818, next occurrence in 2285. Easter on April 25 (latest date): 1943, next occurrence in 2038. See Wikipedia, "List of dates for Easter", and the United States Naval Observatory ephemerides.
5The equinox used for calculating Easter is a fixed ecclesiastical equinox, conventionally placed on March 21, independently of the real astronomical equinox (which most often falls on March 20 in the present Gregorian calendar). See United States Naval Observatory, "The Date of Easter".
6The "paschal full moon" is the full moon of an ecclesiastical moon: a lunar month reconstructed by the computus (Metonic cycle, epacts), whose fourteenth day counts as "full moon." It may differ by one or two days from the astronomical full moon observed in the sky. This convention allows Easter to be fixed in advance, everywhere, without local observation.
7Paques comes from late ecclesiastical Latin Pascha, from Greek Paskha, a transcription (via Aramaic pasḥa) of Hebrew Pessah (פֶּסַח), "Passover." Larousse dictionary, entry "Paques": "du bas latin ecclesiastique Pascha, du grec Paskha, de l'hebreu pesah." The root pasaḥ means "to pass over, to spare."
8Exodus 12:13: "The blood shall be a sign for you, upon the houses where you are; and when I see the blood, I will pass over you, and no plague shall fall upon you to destroy you." Account of the first Passover, on the night of the Exodus. Bible de Jerusalem, Cerf, Paris, 1998.
9Bede the Venerable, De temporum ratione (The Reckoning of Time), ch. 15, "De mensibus Anglorum", completed in 725. Latin text (ed. Ch. W. Jones, 1943, repr. F. Wallis): "Eosturmonath, qui nunc paschalis mensis interpretatur, quondam a dea illorum quae Eostre vocabatur, et cui in illo festa celebrabant, nomen habuit : a cuius nomine nunc paschale tempus cognominant, consueto antiquae observationis vocabulo gaudia novae solemnitatis vocantes." English translation above after Faith Wallis, Bede: The Reckoning of Time, Liverpool University Press, 1999, ch. 15 ("Eosturmonath [...] was once called after a goddess of theirs named Eostre, in whose honour feasts were celebrated in that month").
10The historicity of the goddess Eostre is debated: Bede is the only ancient source to mention her, and several specialists think he may have inferred her existence from the name of the month Eosturmonath alone. In any case, this etymology concerns only the Germanic word Easter / Ostern, never the word Pascha / Paques. See F. Wallis, op. cit., 1999, notes on ch. 15.
11The four Gospels situate the Passion and Resurrection of Christ during the Jewish feast of Passover in Jerusalem: Matthew 26:17 ff.; Mark 14:12 ff.; Luke 22:7 ff.; John 18:28; 19:14. The Resurrection takes place on "the first day of the week" (Mark 16:2; John 20:1), that is, Sunday.
12Saint Paul, 1 Corinthians 5:7-8. Paul applies the vocabulary of the Jewish Passover (sacrificed lamb, unleavened bread, leaven) to Christ in order to describe the Christian paschal mystery. Bible de Jerusalem, Cerf, Paris, 1998.
13The Quartodecimans (from Latin quartodecimani, "of the fourteenth") celebrated Pascha on 14 Nisan, like the Jewish Passover, whatever the day of the week. This usage was widespread in Asia Minor in the second and third centuries. See Catholic Encyclopedia, art. "Easter Controversy" (newadvent.org/cathen/05228a.htm); Eusebius, Hist. eccl., V, 23-25.
14Saint Irenaeus, quoted by Eusebius of Caesarea, Ecclesiastical History, V, 24, 16-17. Irenaeus reports the meeting of Polycarp of Smyrna and Pope Anicetus (c. 154-155): despite their disagreement over the date of Easter, they remained in full communion, Anicetus even yielding the presidency of the Eucharist to Polycarp. Ed. SC 41, Cerf, Paris, 1955.
15Eusebius of Caesarea, Ecclesiastical History, V, 24: Pope Victor I (c. 189-199) attempted to excommunicate the Churches of Asia that remained Quartodeciman. Eusebius specifies that this was an attempt, blocked by the opposition of many bishops. Ed./trans. G. Bardy, Sources chretiennes 41, Cerf, Paris, 1955.
16Eusebius of Caesarea, Ecclesiastical History, V, 24: Irenaeus writes to Pope Victor to remind him that earlier bishops who observed different paschal customs nevertheless remained in communion. The point was not to deny the importance of unity, but to avoid breaking communion brutally over a disciplinary calculation.
17The twenty surviving canons of the First Council of Nicaea (325) contain no canon regulating the calculation of Easter; the paschal decree itself has not been preserved. See Catholic Encyclopedia, art. "Easter Controversy", and the conciliar acts (Mansi, Sacrorum Conciliorum (...) Collectio, t. II).
18Emperor Constantine, letter to the Churches after the Council of Nicaea, reported by Eusebius of Caesarea, Life of Constantine (Vita Constantini), III, 18-19. Constantine insists on unity of date and independence from Jewish usage. Trans. from Patrologia Graeca 20 and the critical edition in Sources chretiennes. Note: Nicaea laid down only the principle of unity, without a detailed formula of calculation.
19The role of the Church of Alexandria as a centre for calculating the date of Easter (which it announced each year, with Rome ratifying) is a post-Nicene custom, not a canon of Nicaea. The paschal tables of Cyril of Alexandria (95 years, 437-531) fixed the Alexandrian 19-year computus that later prevailed in the West.
20The computus designates the set of methods for calculating the date of Easter and the movable feasts. From the early Middle Ages to the Renaissance, it was one of the most demanding branches of science, combining astronomy, arithmetic and calendar calculation. See Arno Borst, The Ordering of Time, University of Chicago Press, 1993.
21The Metonic cycle, attributed to the Athenian astronomer Meton (c. 432 BC), rests on the approximate equality: 19 solar years ≈ 235 synodic months ≈ 6,940 days. After 19 years, the lunar phases fall again almost on the same dates of the solar calendar. Encyclopaedia Britannica, art. "Metonic cycle".
22The golden number (numerus aureus) indicates the rank of a year in the 19-year Metonic cycle: golden number = (year modulo 19) + 1. It helps locate the moon within the cycle. See Catholic Encyclopedia, art. "Epact" (1909).
23The epact is the age of the moon (number of days since the last new moon) on a fixed date of the year (classically January 1). It expresses the excess of the solar year over twelve lunations (about eleven days). The Gregorian computus (after 1582) calculates Easter by epacts, whereas the medieval computus used chiefly the golden number. Catholic Encyclopedia, art. "Epact".
24The Metonic cycle is not exact: the difference between 19 solar years and 235 lunar months produces a drift of about one day every 219 to 250 years. This accumulated inaccuracy justified the correction of the lunar tables during the Gregorian reform of 1582.
25Victorius of Aquitaine, Cursus paschalis (Paschal Cycle), completed in 457 at the request of the Roman archdeacon Hilary (future pope). He proposed a great cycle of 532 years (= 19 x 28), but his tables sometimes gave two competing paschal dates, Latin and Greek. See A. A. Mosshammer, The Easter Computus and the Origins of the Christian Era, Oxford University Press, 2008.
26Dionysius Exiguus, paschal table of 525, prepared at the request of Pope John I. He extended the Alexandrian tables of Cyril (95 dates, 532-626), adopting the Alexandrian 19-year computus rather than that of Victorius. Encyclopaedia Britannica, art. "Dionysius Exiguus".
27When drawing up his paschal table, Dionysius Exiguus refused to count the years from the persecuting emperor Diocletian ("Era of the Martyrs") and chose to begin from the Incarnation: he thus introduced the Christian era "Anno Domini" ("year of the Lord"). Its use then spread through Bede the Venerable. See Wikipedia, art. "Anno Domini"; A. A. Mosshammer, op. cit., 2008.
28Synod of Whitby (664), in Northumbria, convened by King Oswiu: it decided in favour of the Roman (Alexandrian) computus against the Celtic computus (84-year cycle). Principal narrative source: Bede the Venerable, Historia ecclesiastica gentis Anglorum, III, 25. The question of tonsure was also debated.
29Gospel according to Saint Matthew, 16:18-19. At Whitby, the monk Wilfrid won King Oswiu's decision by invoking the authority of the keys entrusted to Saint Peter, to whom Roman usage was attached (Bede, Hist. eccl., III, 25). Bible de Jerusalem, Cerf, Paris, 1998.
30Bede the Venerable, De temporum ratione (The Reckoning of Time), 725: the reference manual of medieval computus, treating the calendar, the lunar cycle and the calculation of Easter. And Historia ecclesiastica gentis Anglorum (c. 731), which spread the Anno Domini era. Trans. F. Wallis, Bede: The Reckoning of Time, Liverpool University Press, 1999.
31The Julian calendar (year of 365.25 days) exceeds the true solar year by about 11 minutes per year, or nearly three days every four centuries. By the sixteenth century, the astronomical equinox fell around March 11, while the computus supposed it to be on the 21st: a ten-day discrepancy had accumulated since Nicaea. See G. V. Coyne, M. A. Hoskin, O. Pedersen (eds.), Gregorian Reform of the Calendar, Specola Vaticana, 1983.
32Gregory XIII, bull Inter gravissimas, dated February 24, 1582, instituting the Gregorian calendar. Latin text and translation on Wikisource. The reform aimed first to correct the calculation of Easter by restoring the equinox to March 21.
33To recover the ten-day delay, the bull prescribed that Thursday, October 4, 1582 be immediately followed by Friday, October 15, 1582, in the Catholic countries that adopted it immediately. The weekly cycle was not interrupted (Thursday -> Friday); only the numbered calendar dates were removed.
34Gregorian rule for leap years: a year divisible by 4 is a leap year, except century years divisible by 100, which are leap years only if also divisible by 400. Thus 1700, 1800, 1900 were common years; 1600 and 2000 leap years. This rule reduces the calendar error to about one day in 3,200 years.
35Aloysius Lilius (Luigi Lilio), Calabrian physician and astronomer (c. 1510-1576), is the author of the underlying scheme of the reform, especially the system of corrected epacts. Having died six years before adoption of the reform, his brother Antonio Lilio submitted his manuscript to the pope. See Wikipedia, art. "Aloysius Lilius"; Gregorian Reform of the Calendar, Specola Vaticana, 1983.
36Christophorus Clavius (1538-1612), German Jesuit and mathematician, member of the pontifical commission, elaborated and publicly defended the calculation of the reform, especially in his Romani calendarii a Gregorio XIII restituti explicatio (1603). See J. M. Lattis, Between Copernicus and Galileo: Christoph Clavius, University of Chicago Press, 1994.
37The Orthodox Churches apply the same Nicene rule (the first Sunday after the full moon after the equinox), but according to the unreformed Julian calendar, whereas the Catholic West has used the Gregorian calendar since 1582. The divergence of dates is therefore calendrical, not doctrinal.
38The difference between the Julian and Gregorian calendars is now thirteen days: Julian March 21 corresponds to Gregorian April 3. The Orthodox reference equinox is therefore thirteen days behind the real astronomical equinox.
39Orthodox Pascha coincides with Latin Easter, or follows it by one, four or five weeks (never two or three), the maximum gap being thirty-five days. See timeanddate.com, "Orthodox Easter", and comparative ephemerides of the two computuses.
40The alleged Orthodox rule that Easter must always fall after the Jewish Passover is not an authentic canon: the so-called Apostolic Canon 7 asked only that Christians not depend on the Jewish calculation. According to several historians, the Byzantine canonist John Zonaras (twelfth century) wrongly inferred a rule from the simple fact that, under the Julian computus, Easter fell after Jewish Passover. Zonaras's commentary edited in G. A. Rhalles – M. Potles, Σύνταγμα τῶν θείων καὶ ἱερῶν κανόνων, Athens, 1852-1859. See A. S. Damick, "Why Is Orthodox Easter on a Different Day?", Ancient Faith, 2022; and the study "The Image of Jews According to the Canonical Tradition of the Orthodox Church", Religions 14/1 (2023), which identifies Zonaras as the first Byzantine author to interpret the so-called Apostolic Canon 7 in this way (mdpi.com/2077-1444/14/1/91).
41In 2025, Catholics and Orthodox celebrated Easter on the same day, Sunday, April 20, 2025, by coincidence of the two computuses. Catholic News Agency / EWTN, April 2025.
42Catholic and Orthodox Easter dates coincide several times each decade. After 2025, the next coincidences will occur in 2028 (April 16), 2031 (April 13) and 2034 (April 9). See timeanddate.com, comparison of Western and Eastern Easter.
43Second Vatican Council, Sacrosanctum Concilium (constitution on the sacred liturgy, December 4, 1963), Appendix: "Declaration of the Second Ecumenical Council of the Vatican on Revision of the Calendar." The council is not opposed to a fixed date of Easter in the Gregorian calendar, with the assent of those concerned by this question, especially the brethren who are not in communion with the Apostolic See. Quoted from the official English text on vatican.va.
44Aleppo consultation (Aleppo, Syria), March 5-10, 1997, organized by the World Council of Churches (Faith and Order) and the Middle East Council of Churches: declaration "Towards a Common Date for Easter." Proposal: keep the norm of Nicaea, but calculate equinox and full moon according to modern astronomy, using the meridian of Jerusalem. It was never adopted for lack of agreement. Text on oikoumene.org.
45On June 12, 2015, before the priests gathered at Saint John Lateran for the world retreat of priests, Pope Francis declared that an agreement had to be reached ("we have to come to an agreement") on a common date for Easter, joking: "When did Christ rise from the dead? ... My Christ rose today, and yours next week", and calling this disunity a scandal ("this disunity is a scandal"). Remarks reported by Catholic News Agency, "Will Pope Francis change the date of Easter?", June 19, 2015 (catholicnewsagency.com/news/32189); the event appears in the Vatican calendar (vatican.va, Mass of June 12, 2015 on the occasion of the world retreat of priests). The pope restated this openness on several occasions, notably on November 19, 2022, receiving Catholicos-Patriarch Mar Awa III: "we are ready to accept any proposal that is made together" (address on vatican.va, 20221119-patriarca-chiesa-assira).
46The year 2025 marked the 1700th anniversary of the Council of Nicaea (325-2025) and, by coincidence, saw Catholics and Orthodox celebrate Easter on the same day (April 20). This conjunction was presented as a providential opportunity to progress toward a common date. The International Theological Commission devoted a document to the jubilee of Nicaea dated April 3, 2025 (vatican.va).
47Gospel according to Saint Matthew, 5:17: "Think not that I have come to abolish the law and the prophets; I have come not to abolish them but to fulfil them." The preservation of a lunar reference (inherited from the Jewish Passover) for fixing Easter illustrates this relationship of fulfilment between the Old and New Covenants. Bible de Jerusalem, Cerf, Paris, 1998.
48Catechism of the Catholic Church, §§ 1166-1170: Sunday, the "first day of the week", memorial of the Resurrection, is the day par excellence of the liturgical assembly; § 1170 concerns the fixing of Easter's date at the Council of Nicaea. Eastertide is the summit of the liturgical year.
49Exsultet (Easter Proclamation), sung by the deacon at the Easter Vigil before the paschal candle (Roman Missal, Easter Vigil). Latin text: "Haec nox est, in qua, destructis vinculis mortis, Christus ab inferis victor ascendit"; official English Missal: "This is the night, when Christ broke the prison-bars of death and rose victorious from the underworld". The English rendering above is a paraphrase, not a literal quotation of the Missal.
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