Old and New Calendars
Since the Julian Calendar had been in continuous use in the Christian East and West throughout the centuries, the subsequent introduction of the Gregorian Calendar in the West created yet another anomaly in the deteriorating relations between the two Churches. The need for correction of the Julian Calendar was well understood in the East and had even led some to devise a new calendar themselves. Nevertheless, the Julian Calendar remained in use throughout the Byzantine period and beyond. Despite the efforts of the emissaries of Pope Gregory to convince the Orthodox to accept the New (Gregorian) Calendar, the Orthodox Church rejected it. The main reason for its rejection was that the celebration of Easter would be altered: contrary to the injunctions of canon 7 of the Holy Apostles, the decree of the First Ecumenical Synod, and canon 1 of Ancyra, Easter would sometimes coincide with the Jewish Passover in the Gregorian calendar.
This is where the matter stood until the end of World War I. Until then, all Orthodox Churches had strictly abided by the Old (Julian) Calendar, which at present is 13 days behind the New Calendar long since adopted by the rest of Christendom. In May of 1923, however, an "Inter-Orthodox Congress" was convened at Constantinople by the then Ecumenical Patriarch, Meletios IV. Not all Orthodox Churches were in attendance. The Churches of Serbia, Romania, Greece, and Cyprus were; the Churches of Alexandria, Antioch and Jerusalem, although invited, were not; the Church of Bulgaria was not invited. Several issues were under discussion at the congress, one of which was the adoption of the New Calendar. No unanimous agreement was reached on any of the issues discussed. Several of the Orthodox Churches, however, did eventually agree, though not all at the same time, to adopt the New Calendar. These were the Churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, and most recently, Bulgaria (1968); on the other hand, the Churches of Jerusalem, Russia and Serbia, along with the monasteries on Mt. Athos, all continue to adhere to the Old Calendar.
The determination of the date of Easter is governed by a computation based on the vernal equinox and the phase of the moon. According to the ruling of the First Ecumenical Synod in 325, Easter Sunday should fall on the Sunday which follows the first full moon after the vernal equinox. If the full moon happens to fall on a Sunday, Easter is observed the following Sunday. The day taken to be the invariable date of the vernal equinox is March 21.
The other difference in the determination of Easter between the Orthodox and other Christian Churches concerns the date of Passover. Jews originally celebrated Passover on the first full moon following the vernal equinox. Christians, therefore, celebrated Easter on the first Sunday after the first full moon following the vernal equinox. After the destruction of Jerusalem in 70 A.D. and the other tragic events which gave rise to the dispersal of the Jews, Passover sometimes preceded the vernal equinox. This was occasioned by the dependence of the dispersed Jews upon local pagan calendars for the calculation of Passover. As a consequence, most Christians eventually ceased to regulate the observance of Easter by the Jewish Passover. Their purpose, of course, was to preserve the original practice of celebrating Easter following the vernal equinox.
As an alternative to calculating Easter by the Passover, "paschal (Easter) cycles" were devised. The Orthodox Church eventually adopted a 19-year cycle, the Western Church an 84-year cycle. The use of two different "paschal cycles" inevitably gave way to differences between the Eastern and Western Churches regarding the observance of Easter. Varying dates for the vernal equinox increased these differences. Consequently, it is the combination of these variables which accounts for the different date of Orthodox Easter, whenever it varies from the rest of Christendom.