- A question of no small importance arose at that time [i.e. the time of Pope Victor I, about A.D. 190]. The dioceses of all Asia [meaning the Roman Province of Asia, see also Early Christianity#Western Anatolia], according to an ancient tradition, held that the fourteenth day of the moon [of Nisan], on which day the Jews were commanded to sacrifice the lamb, should always be observed as the feast of the life-giving pasch [epi tes tou soteriou Pascha heortes], contending that the fast ought to end on that day, whatever day of the week it might happen to be. However it was not the custom of the churches in the rest of the world to end it at this point, as they observed the practice, which from Apostolic tradition has prevailed to the present time, of terminating the fast on no other day than on that of the Resurrection of our Saviour.
Quartodecimanism, a word not used in Eusebius' account, refers to the practice of fixing the celebration of Passover for Christians on the fourteenth (Latin quarta decima) day of Nisan in the Old Testament's Hebrew Calendar (for example ). This was the original method of fixing the date of the Passover, which is to be a "perpetual ordinance". According to the Gospel of John (for example ), this was the day that Jesus was crucified in Jerusalem. (The Synoptic Gospels place the day on 15 Nisan, see also Chronology of Jesus.)
A letter of St. Irenaeus shows that the diversity of practice regarding Easter had existed at least from the time of Pope Sixtus I (c. 120). Further, Irenaeus states that St. Polycarp kept Easter on the fourteenth day of the moon, whatever day of the week that might be, following therein the tradition which he claimed to have derived from St. John the Apostle.
About 195, Pope Victor I attempted to excommunicate the Quartodecimans, turning the divergence of practice into a full-blown ecclesiastical controversy. According to Eusebius, synods were convened and letters were exchanged. The controversy seems to have died down after Irenaeus and other bishops reminded Victor of his predecessor Anicetus' more tolerant attitude.
The second stage in the Easter controversy centres round the First Council of Nicaea (A.D. 325). Granted that the great Easter festival was always to be held on a Sunday, and was not to coincide with a particular age of the moon, which might occur on any day of the week, a new dispute arose as to the determination of the Sunday itself. Shortly before the Nicean Council, in 314, the Provincial council of Arles in Gaul had maintained that the Lord's Pasch should be observed on the same day throughout the world and that each year the Bishop of Rome should send out letters setting the date of Easter.
The Syrian Christians always held their Easter festival on the Sunday after the Jews kept their Pesach. On the other hand at Alexandria, and seemingly throughout the rest of the Roman Empire, the Christians calculated the time of Easter for themselves, paying no attention to the Jews. In this way the date of Easter as kept at Alexandria and Antioch did not always agree. The Jewish communities in some places, possibly including Antioch, used methods of fixing their month of Nisan that sometimes put the 14th day of Nisan before the spring equinox. The Alexandrians, on the other hand, accepted it as a first principle that the Sunday to be kept as Easter Day must necessarily occur after the vernal equinox.
The Council of Nicaea ruled that all churches should follow a single rule for Easter, which should be computed independently of the Jewish calendar, as at Alexandria. However, it did not make any explicit ruling about the details of the computation, and it was several decades before the Alexandrine computations stabilized into their final form, and several centuries beyond that before they became normative throughout Christendom.
The Roman missionaries coming to Britain in the time of St. Gregory the Great found the British Christians, and the Irish missionaries who evangelized the English from the north, adhering to a system of Easter computation which differed from that used in the Mediterranean world. This British and Irish system, on the evidence of Bede, fixed Easter to the Sunday falling in the seven-day period from the 14th to the 20th of its lunar month, according to an 84-year cycle. The limits of luna 14-luna 20 are corroborated by Columbanus.  The 84-year cycle, the lunar limits, and an equinox of March 25 also receive support from MacCarthy's analysis of Padua, Biblioteca Antoniana, MS I.27.  Any of these features alone could have led to occasional discrepancies from the date of Easter as computed by the Alexandrine method.
This 84-year cycle (called the latercus) gave way to the Alexandrine computus in stages. The Alexandrine computus may have been adopted in parts of the south of Ireland in the first half of the 7th century. Among the northern English, the use of the Alexandrine computus over the Brittano-Irish cycle was decided at the Synod of Whitby in AD 664. The Alexandrine computus was finally adopted by the Irish colonies in northern Britain in the early 8th century.
After the promulgation of the Gregorian calendar in 1582, the Catholic and Protestant churches of the West came to follow a different method of computing the date of Easter from the one that had been previously accepted. Most Eastern Orthodox churches continued to follow the older practice and this difference has continued to the present time, despite several attempts to achieve a common method for computing the date of Easter. In 1997 the World Council of Churches proposed a reform of the method of determining the date of Easter at a summit in Aleppo, Syria: Easter would be defined as the first Sunday following the first astronomical full moon following the astronomical vernal equinox, as determined from the meridian of Jerusalem. The reform would have been implemented starting in 2001, since in that year the Eastern and Western dates of Easter would coincide. This reform has not yet been implemented.
- ↑ Exodus 12:14 NRSV
- ↑ Charles Jones, Bedae Opera de temporibus, (Cambridge, Mediaeval Academy of America), 1943, p. 25.
- ↑ Bede, Church History of the English People, 2.2, in J.E. King, tr., Bede: Historical Works, Vol. 1, Loeb Classical Library, Cambride, 1930, p. 205.
- ↑ Columbanus, Letter to Pope Gregory, in A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume 13, p. 40.
- ↑ MacCarthy, Daniel, "Easter Principles and a Fifth-Century Lunar Cycle used in the British Isles", Journal for the History of Astronomy, 24, 204-224 (1993).
- ↑ Cummian, Letter on the Easter Controversy, PL 87.969.
- ↑ Bede, Church History, 3.25.
- ↑ Bede, Church History, 5.22.
- ↑ http://wcc-coe.org/wcc/what/faith/easter.html
- Jones, Charles W. Bedae Opera de Temporibus. Cambridge: Mediaeval Academy of America, 1943. pp. 3-104.
- McCarthy, Daniel. "The Origin of the Latercus Paschal Cycle of the Insular Celtic Churches", Cambrian Medieval Celtic Studies, 28(1994): 25-49.
- Mosshammer, Alden A. The Easter Computus and the Origins of the Christian Era. Oxford: Oxford University Press, 2008. ISBN 0-19-954312-7.
- Ó Cróinín, Dáibhí and Daniel McCarthy. "The 'Lost' Irish 84-year Easter Table Rediscovered", Peritia, 6-7(1987-88): 227-242.
- Walsh, Maura and Dáibhí Ó Cróinín. Cummian's Letter De controversia paschali and the De ratione conputandi. Toronto: Pontifical Institute of Medieval Studies, 1988.
- Wallis, Faith. Bede: The Reckoning of Time. Liverpool: Liverpool University Press, 2004. pp. xxxiv-lxiii.
- Catholic Encyclopedia: "Easter Controversy"
- Philip Schaff's History of the Christian Church, volume 3, section 79: "The Time of the Easter Festival"