How Eastern Orthodox Easter is Set
The observance of Easter by Eastern Orthodox Churches varies from the date that other Christian Churches celebrate Easter in most yeas. This is due not only to calendar differences, but also to the relationship with the Jewish Passover.
The first month of the old ecclesiastical calendar of the Jews is the month of Nisan, which begins with the new moon occurring about the time of the vernal equinox, the beginning of spring.
The 14th of Nisan marks the beginning of the holy season of the Jewish Passover, commemorating the sparing of the Hebrews in Egypt when God, smiting the first born in Egyptian homes, passed over the houses of the Children of Israel whose doorposts were marked by sprinkling them with the blood of a lamb. During this period of the Jewish year, Jesus was crucified. The Last Supper, of which Christ partook, with his twelve disciples, was the Feast of the Passover. Therefore the Jewish Passover and the Christian Easter are closely connected.
The rule for determining the date for Easter was set by the Council of Nicaea in 325 A.D. According to this rule, Easter must be observed on the first Sunday after the first spring’s full moon. That is why Easter cannot fall earlier than March 23rd. or later than April 25th under the old Julian Calendar. The Council of Nicaea wanted to keep Easter forever separated from the Jewish Passover, so it was decided that when Easter and Passover itself came on the same Sunday, the Christian Easter would be observed a week later. Easter in the Eastern Orthodox Churches is never celebrated before the Jewish Passover because Christ Himself observed the Passover at the Last Supper before His Betrayal.