Moveable Feasts
Pascha is, by far, the most important day in the ecclesiastical year, and all other days, in one way or another, are dependent upon it. Pascha falls on different calendar dates from year to year, calculated according to a strict set of rules (see Computus for details). While the Fixed Cycle begins on September 1, the new Paschal Cycle begins on "Zaccheus Sunday" (the beginning of the preparatory season before Great Lent), eleven Sundays before Pascha, and continues until the Zaccheus Sunday of the following year. The Epistle and Gospel readings at the Divine Liturgy throughout the year are determined by the date of Pascha.
Read more about this topic: Eastern Orthodox Liturgical Calendar