The Date of Pascha.

The time of the Great Lent is dependent on the date of Pascha, which varies from year to year. According to a Canon of the First Ecumenical Council (Nicea — 325), Holy Pascha is the first Sunday after the first Full Moon which falls upon or immediately after the Spring Equinox (according to ancient reckoning — March 21). In addition, this Council decreed that Pascha cannot precede or fall on the Jewish Passover (14th day of the Month Nissan). The Full Moon used for the purposes of calculating the date of Pascha is the fourteenth day of a Lunar Month reckoned according to an ancient ecclesiastical computation and is not the actual astronomical Full Moon.

The number of days between each Full Moon (the Lunar Month) is not exact according to the Solar Calendar (usually about 29V& days) and ancient calendars added or subtracted a period called an epact to harmonize the Lunar and Solar Calendars. These epacts as calculated by the Orthodox Church, vary from those calculated by the Western Churches. In addition, the Western Churches do not follow the Nicean Council's decree that Pascha must not precede or fall on the Jewish Passover, and it is for these reasons that there is often a great variance from one year to the next between the Orthodox Church and the Western Churches concerning the date of Holy Pascha.