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.
Herein lies the first difference in the determination of Easter between the Orthodox Church and the other Christian Churches. The Orthodox Church continues to base its calculations for the date of Easter on the Julian Calendar, which was in use at the time of the First Ecumenical Synod. As such, it does not take into consideration the number of days which have since then accrued due to the progressive inaccuracy of the Julian Calendar. In other words, a difference of 13 days exists between the accepted date for the vernal equinox then and now. In the West, this discrepancy was addressed in the 16th century through the adoption of the Gregorian Calendar, which adjusted the Julian Calendar still in use by all Christians at that time.
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.
