The Major Feasts of the Church

The ecclesiastical year, which according to Byzantine practice begins on the first of September, is divided between movable and immovable or fixed holy days. The movable holy days are determined by the date of Easter, the most important of all feast days, which is in a class by itself. The determination of the date of Easter was definitively regulated by the decision of the First Ecumenical Synod, held in Nicaea (325). Next in importance to Easter are the “twelve great feasts,” of which three are movable. Eight of these feasts are devoted to Christ and four to the Virgin Mary. There are also a number of feast days of varying importance, most of which commemorate the more popular saints.

Link: http://www.goarch.org/en/ourfaith/articles/article8713.asp