While western Christians have completed their celebrations of the Lenten season and Easter, eastern Christians are just gearing up for their own. How did that happen? How did the eastern and western Easters diverge? Curiously, this had something to do with the Roman empire before the advent of Christianity, and much to do with what happened in the empire later on.
Generally speaking, one of the early challenges for the early Christians was that of mapping their new religion onto older Roman institutions. For some of their early festivals, they simply stapled them on to Roman holidays of great popularity and long duration. That's how Christmas came to be associated with and overlapped with the Roman Saturnalia. According to the evidence provided by the Venerable Bede, the feast that his parishioners called "Easter" even took its name from an Anglo-Saxon springtime goddess named Eostre.
That said, Christmas and Easter are profoundly different holidays in one important way: Christmas always takes place on the same day, whereas Easter does not. Several important Christian feasts always take place on the same day: Epiphany on Jan. 6, the Annunciation on March 25, the Assumption of the Virgin on Aug. 15 and Christmas on Dec. 25.
But most other Christian festivals were moveable feasts, and all were calculated in relation to Easter, which also moves. The feast was called Pascha in Greek (deriving from the Greek word for suffering, paschein), and was the most popular and the most important religious holiday in the Early Church.
The Early Christians did two things at their Easter celebrations, primarily: they commemorated the miraculous rising of Jesus after his crucifixion; and they welcomed new Christians into the Church. The 40-day preparation for the Easter feast intensified during Holy Week, and culminated in an all-night vigil on Holy Saturday. The catechumens, those who had received instruction in anticipation of joining the Christian Church, were baptized on Easter morning, then offered communion for the very first time.
But when was this done? It is important to recall that Jesus had gone to Jerusalem to celebrate the Passover when he was killed, a Jewish feast commemorating the Hebrews' deliverance from enslavement in Egypt. So the first question was whether followers of Jesus had simply to celebrate Passover, as their Lord had done, or to distinguish their celebration somehow from Passover. In the early period of Christian formation, some followers of Jesus actually celebrated Easter according to Jewish custom, on a fixed day in the Jewish lunar calendar (the 14th of Nisan). Others felt that this Christian feast should be clearly differentiated from Passover, by weeks if not days.
It is also important to recall that there was not one central religious administration in this period, when such important Christian questions were being debated. Here once again, the Christians mapped their administrative structures onto existing structures in the Roman empire. An important Roman city was an important Christian city, too.
Five administrative centers emerged as especially important Christian centers (called patriarchates) by the late fourth century: Rome, Constantinople, Antioch, Caesarea and Alexandria. Prior to the foundation of the city of Constantinople in 330 CE, Rome and Antioch had emerged as the most important western and eastern Christian centers, respectively. Alexandria in Egypt was positioned somewhere in the middle.
An important piece of evidence for the wide variation in Christian computations of the date of Easter is Saint Augustine, who reported with some dismay in 387 C.E. that Easter was celebrated in Gaul on March 21, in Rome on April 18 and in Alexandria on April 25.
They spoke Greek in Alexandria, and they spoke Latin in Rome. And here's the strange thing: They used different calendars as well. The Greek emperor, Ptolemy II (309-246 B.C.E.), had authorized the creation of a new calendar in Alexandria. The point was to create a system whereby the major solar events like solstices and equinoxes came on the same day every year. But as with most ancient calendars, the days got out of sync.
In Rome, Julius Caesar arranged for the correction of this Ptolemaic Greek calender in 46 B.C.E.; it became known as the Julian calendar. It, too, was imperfect. By the time the Emperor Constantine converted to Christianity in 312 C.E., the Julian calendar was four days out of sync (it is 13 days out of sync today).
And so, in 1582, Pope Gregory XIII (1502-1585) commissioned the new class of papal astronomers, who now had a much more thorough understanding of the way the earth and the sun were related, to correct the Julian calendar. They decided that the easiest way to correct the Julian calendar was through a disarming two-fold process: first, they simply added 10 days, such that Friday, Oct. 5 became Friday, Oct. 15 in 1582; second, they opted to add one day to the end of February once every four years. That's how we got the "leap year."
This is the so-called Gregorian calendar. And not surprisingly, most Protestants, such as the Puritans in the colonial period in North America, did not accept this change for quite some time; it was the Pope's calendar, after all. (George Washington celebrated his birthday on Feb. 11 according to the Julian calendar, but we celebrate it according to the Gregorian calendar on February 22). England and her North American colonies finally converted to the Gregorian calendar in 1752.
Greece was the last nominally Orthodox country to accept the Gregorian calendar in 1923, but strictly for secular purposes. In 1924, the Ecumenical Patriarch in Istanbul called for the adoption of the Gregorian calendar for fixed Christian feasts like Christmas, but not all Orthodox churches agreed even to this. Thus, in the interest of the liturgical unity of the Orthodox churches, the moveable Christian feasts like Easter and the rest were still calculated according to the Julian calendar.
Traditionally, Christians have celebrated the Easter feast on the first Sunday after the "Paschal Moon," the first full moon following the vernal equinox, the date on which day and night are each 12 hours long. That's why Easter always falls between March 22 and April 25. However, the Eastern Orthodox Easter feast is required to take place after the Jewish feast of Passover is completed.
In 2012, the first full moon after the vernal equinox on March 20-21 (a fixed date in the Gregorian calendar) appeared on Friday, April 6, so the western churches celebrated their Easter last Sunday. But the Jewish feast of Passover was also being celebrated that same weekend, and so through a complicated navigation between the Julian and Gregorian calendars, the Orthodox churches will celebrate their Easter this coming Sunday.
Calendars create perceptions and realities to match in much the same way that maps do. Calendars organize time the way that maps organize space. The early Christian Church was very much concerned to leave its mark on the way Christians perceived both space and time. And that is why the politics of the computation of even so central and unifying a feast as Easter can remain so divisive.