In 725, Bede succinctly wrote, “The Sunday following the full Moon which falls on or after the equinox will give the lawful Easter.” However, this does not reflect the actual ecclesiastical rules precisely. One reason for this is that the full moon involved (called the Paschal full moon) is not an astronomical full moon, but the 14th day of a calendar lunar month. Another difference is that the astronomical equinox is a natural astronomical phenomenon, which can fall on 19, 20 March, or 21, while the ecclesiastical date is fixed by convention on 21 March.
In applying the ecclesiastical rules, Christian churches use 21 March as the starting point in determining the date of Easter, from which they find the next full moon, etc. The Eastern Orthodox and Oriental Orthodox Churches continue to use the Julian calendar. Their starting point in determining the date of Orthodox Easter is also 21 March, but according to the Julian reckoning, which currently corresponds to 3 April in the Gregorian calendar. In addition, the lunar tables of the Julian calendar are four days (sometimes five days) behind those of the Gregorian calendar. The 14th day of the lunar month according to the Gregorian system is only the ninth or tenth day according to the Julian. The result of this combination of solar and lunar discrepancies is divergence in the date of Easter in most years (see table).
Easter is determined on the basis of lunisolar cycles. The lunar year consists of 30-day and 29-day lunar months, generally alternating, with an embolismic month added periodically to bring the lunar cycle into line with the solar cycle. In each solar year (1 January to 31 December inclusive), the lunar month beginning with an ecclesiastical new moon falling in the 29-day period from 8 March to 5 April inclusive is designated as the paschal lunar month for that year. Easter is the third Sunday in the paschal lunar month, or, in other words, the Sunday after the paschal lunar month’s 14th day. The 14th of the paschal lunar month is designated by convention as the Paschal full moon, although the 14th of the lunar month may differ from the date of the astronomical full moon by up to two days. Since the ecclesiastical new moon falls on a date from 8 March to 5 April inclusive, the paschal full moon (the 14th of that lunar month) must fall on a date from 21 March to 18 April inclusive.
The Gregorian calculation of Easter was based on a method devised by the Calabrian doctor Aloysius Lilius (or Lilio) for adjusting the epacts of the moon, and has been adopted by almost all Western Christians and by Western countries who celebrate national holidays at Easter. For the British Empire and colonies, a determination of the date of Easter Sunday using Golden Numbers and Sunday letters was defined by the Calendar (New Style) Act 1750 with its Annexe. This was designed to exactly match the Gregorian calculation.
The precise date of Easter has at times been a matter for contention. By the later 2nd century, it was accepted that the celebration of the holiday was a practice of the disciples and an undisputed tradition. The Quartodeciman controversy, the first of several Easter controversies, then arose concerning the date on which the holiday should be celebrated.
The term “Quartodeciman” refers to the practice of celebrating Easter on Nisan 14 of the Hebrew calendar, “the LORD’s passover” (Leviticus 23:5). According to the church historian Eusebius, the Quartodeciman Polycarp (bishop of Smyrna, by tradition a disciple of John the Evangelist) debated the question with Anicetus (bishop of Rome). The Roman province of Asia was Quartodeciman, while the Roman and Alexandrian churches continued the fast until the Sunday following (the Sunday of Unleavened Bread), wishing to associate Easter with Sunday. Neither Polycarp nor Anicetus persuaded the other, but they did not consider the matter schismatic either, parting in peace and leaving the question unsettled.
Controversy arose when Victor, bishop of Rome a generation after Anicetus, attempted to excommunicate Polycrates of Ephesus and all other bishops of Asia for their Quartodecimanism. According to Eusebius, a number of synods were convened to deal with the controversy, which he regarded as all ruling in support of Easter on Sunday. Polycrates (circa 190), however, wrote to Victor defending the antiquity of Asian Quartodecimanism. Victor’s attempted excommunication was apparently rescinded and the two sides reconciled upon the intervention of bishop Irenaeus and others, who reminded Victor of the tolerant precedent of Anicetus.
Quartodecimanism seems to have lingered into the 4th century, when Socrates of Constantinople recorded that some Quartodecimans were deprived of their churches by John Chrysostom and that some were harassed by Nestorius.
It is not known how long the Nisan 14 practice continued. But both those who followed the Nisan 14 custom, and those who set Easter to the following Sunday had in common the custom of consulting their Jewish neighbors to learn when the month of Nisan would fall, and setting their festival accordingly. By the later 3rd century, however, some Christians began to express dissatisfaction with the custom of relying on the Jewish community to determine the date of Easter. The chief complaint was that the Jewish communities sometimes erred in setting Passover to fall before the Northern Hemisphere spring equinox. The Sardica paschal table confirms these complaints, for it indicates that the Jews of some eastern Mediterranean city (possibly Antioch) fixed Nisan 14 on dates well before the spring equinox on multiple occasions.
Because of this dissatisfaction with reliance on the Jewish calendar, some Christians began to experiment with independent computations.[nb 6] Others, however, felt that the customary practice of consulting Jews should continue, even if the Jewish computations were in error.
This controversy between those who advocated independent computations, and those who wished to continue the custom of relying on the Jewish calendar, was formally resolved by the First Council of Nicaea in 325, which endorsed the move to independent computations, effectively requiring the abandonment of the old custom of consulting the Jewish community in those places where it was still used. Epiphanius of Salamis wrote in the mid-4th century:
… the emperor … convened a council of 318 bishops … in the city of Nicea … They passed certain ecclesiastical canons at the council besides, and at the same time decreed in regard to the Passover that there must be one unanimous concord on the celebration of God’s holy and supremely excellent day. For it was variously observed by people …
Some scholars have concluded that no detailed method of determining the date of Easter was specified by the Council. In any case, in the years following the council, the computational system that was worked out by the church of Alexandria came to be normative. It took a while for the Alexandrian rules to be adopted throughout Christian Europe, however. The Church of Rome continued to use an 84-year lunisolar calendar cycle from the late 3rd century until 457. It then switched to an adaptation by Victorius of Aquitaine of the Alexandrian rules. Because this Victorian cycle differed from the Alexandrian cycle in the dates of some of the Paschal Full Moons, and because it tried to respect the Roman custom of fixing Easter to the Sunday in the week of the 16th to the 22nd of the lunar month (rather than the 15th to the 21st as at Alexandria), by providing alternative “Latin” and “Greek” dates in some years, occasional disagreements from the date of Easter as fixed by Alexandrian rules continued. The Alexandrian rules were adopted in their entirety in the 6th century. From this time, therefore, all disputes between Alexandria and Rome as to the correct date for Easter cease, as both churches were using identical tables.
Early Christians in Britain and Ireland also used an 84-year cycle. From the 5th century onward this cycle set its equinox to 25 March and fixed Easter to the Sunday falling in the 14th to the 20th of the lunar month inclusive. This 84-year cycle was replaced by the Alexandrian method in the course of the 7th and 8th centuries. Churches in western continental Europe used a late Roman method until the late 8th century during the reign of Charlemagne, when they finally adopted the Alexandrian method. Since 1582, when the Catholic Church adopted the Gregorian calendar while the Eastern Orthodox and most Oriental Orthodox Churches retained the Julian calendar, the date on which Easter is celebrated has again differed.
The Greek island of Syros, whose population is divided almost equally between Catholics and Orthodox, is one of the few places where the two Churches share a common date for Easter, with the Catholics accepting the Orthodox date – a practice helping considerably in maintaining good relations between the two communities.