THE ORTHOOOX RECKONING OF THE DATE OF EASTER
Both East and West reckon Easter on the first Sunday after the first full moon following the spring equinox (which falls on Match 21). The Orthodox, however, observe the ruling of the First Ecumenical Council that Easter must always fall AFTER the Jewish Passover, so as to preserve the biblical sequence of the Crucifixion and the Resurrection. Thus, Holy Pascha must always fulfill, and therefore follow, the Hebrew Passover.
Because of this, and because the Julian (Old Style) calendar used by the Orthodox to reckon Easter differs by about thirteen days from the Gregorian calendar used In the West, the Easter of the Orthodox and that of the Western Churches do not usually coincide.
Sometimes, Western Easter may precede the Jewish Passover; however, the Orthodox method
never permits this, which is in agreement with the ancient practices of the early Church.