Our calendar is a Christian calendar. Its years are counted from the birth of Christ, celebrated annually on December 25th. The other great event in the Christian faith, greater even than the Nativity, is the Crucifixion and the Resurrection of Christ. This is the event which gives hope and meaning to every Christian.
The Crucifixion and Resurrection are celebrated each year at Easter. In the early Church, these were the events which led Christianity irrevocably away from Judaism, and for almost 700 years, the date of Easter was the subject of debate, disagreement and potential schism.
In the Gospels, the events of Holy Week took place during Passover, the Jewish festival which commemorated the exodus of the Israelites from Egypt when the angel of death "passed over" the homes of the Hebrews and spared the first-born. In the Jewish calendar, Passover took place on the fourteenth day of the month of Nisan.
The Jewish calendar was a lunar calendar in which the beginning of the month was marked by the new crescent Moon, so the fourteenth day corresponded to the Full Moon. Moreover, Nisan was the first month of the Jewish year, which was arranged so that the new year began at approximately the Spring Equinox.
The early Christians, remembering their Jewish roots, continued to celebrate Easter at the time of Passover. They, like the Jewish community itself, could not say in advance when Passover would fall. This decision was the prerogative of the Sanhedrin in Jerusalem and its successors, who kept the method of determining Passover a closely-guarded secret for centuries.
By the third century A.D., the Christian churches had rejected the authority of the Jewish councils to decide the date of Passover and had begun to calculate tables of the date of Easter for themselves. This, however, led to internal disagreement. There were two diametrically opposite views. One group were still strongly influenced by the Jewish tradition that the Passover must fall on the fourteenth day of the lunar month. Their insistence on the significance of the number 14 led to them being named Quartodecimians.
The other group believed that the celebration of Easter should follow the events of Holy Week, in which the Crucifixion occurred on Friday and the Resurrection on the following Sunday. They held that Easter Day must be a Sunday, regardless of the day of the month.
The Eastern Church observed Easter on the fourteenth day of the month. The Western Church observed Easter on Sunday. This quarrel threatened to lead to schism, and it was one of the reasons which led Constantine the Great to summon the leaders of both Eastern and Western churches to the Council of Nicaea in A.D. 325. This Council is best-remembered for the Nicene Creed, the great statement of Christian belief, but it also agreed the formula for determining the date of Easter.
The Council decreed that Easter should be the first Sunday after the Full Moon following the Spring Equinox, March 21st, but if that Full Moon fell on a Sunday, then Easter should be the Sunday after.
The final phrases hint at the depth of the disagreement, for without them, it was possible that Easter might be celebrated at the time of the Full Moon, the fourteenth day of the lunar month, which was the Quartodecimian view.
Even after the Council of Nicaea, the matter was not yet settled. There remained the problem of how to predict the date of the Full Moon. The astronomers knew of at least four different cycles which linked the lunar month with the year. There was the Greek cycle which equated 8 years to 99 lunar months. There was the Metonic cycle which made 19 years equal to 235 lunar months. The Roman cycle matched 84 years to 1039 lunar months. Finally, the cycle devised by Victorius in A.D. 457 took the 19-year Metonic cycle and the 28-year cycle of days of the week within the Julian calendar and made a cycle of 532 years.
Rome used the 532-year Victorian cycle, but the church in Britain and Ireland, which had always looked first to its Celtic roots, preferred the older Roman cycle of 84 years. For two centuries, Britain and Ireland celebrated Easter on a different Sunday to Rome in certain years.
The conflict came to a head in A.D. 664 in the small fishing town of Whitby on the coast of east Yorkshire. There, at the Synod of Whitby, Wilfrid, Archbishop of York, tried to persuade Oswy, King of Mercia, to reject the 28-year cycle. In the end, Oswy announced that he would rather accept the authority of Saint Peter over that of Columba, saying: "Then will I rather obey the porter of Heaven, lest when I reach its gates, he who has the keys in his keeping turn his back on me, and there be none to open." Oswy wisely did not want to jeopardise his place in Heaven. Rome had won.