People often wonder why the dates of Easter and Ash Wednesday and other feasts are different each year. These are “moveable feasts” that are fixed by the cycle of the Moon’s phases. Easter (or properly, Pascha) is essentially defined to be the first Sunday after the first Full Moon after the first day of spring, which is different every year. Ash Wednesday is defined to be 40 days ahead of the pre-calculated date of Easter.
The astronomical basis of Easter, and its correspondence to the Biblical feast of Passover in the Hebrew lunar calendar, is explain in our Signs & Seasons curriculum.