Twelve Great Feasts

In the Eastern Orthodox Church, the feast of the Resurrection of Jesus, called Pascha (Easter), is the greatest of all holy days and as such it is called the "feast of feasts". Immediately below it in importance, there is a group of Twelve Great Feasts (Greek: Δωδεκάορτον). Together with Pascha, these are the most significant dates on the Orthodox liturgical calendar. Eight of the great feasts are in honor of Jesus Christ, while the other four are dedicated to the Virgin Mary — the Theotokos.

The Twelve Great Feasts are as follows (note that the liturgical year begins with the month of September):

Besides the Twelve Great Feasts, the Orthodox Church knows five other feasts that rank as great feasts, yet without being numbered among the twelve. They are: the Circumcision of Christ (1 January), the Nativity of St. John the Baptist (24 June), the Feast of Saints Peter and Paul (29 June), the Beheading of St John the Baptist (29 August), and the Protecting Veil of the Theotokos (1 October).

Mary was born to elderly and previously barren parents by the names of Joachim and Anna (now saints), in answer to their prayers. Orthodox Christians do not hold to the Roman Catholic doctrine of the Immaculate Conception of Mary, in which it is taught that Mary was preserved from the ancestral sin that befalls us all as descendants of Adam and Eve, in anticipation of her giving birth to the sinless Christ. The Orthodox believe that Mary, and indeed all mankind, was born only to suffer the consequences of the ancestral sin (being born into a corrupt world surrounded by temptations to sin), the chief of which was the enslavement to Death, and thus needed salvation from this enslavement, like all mankind. The Roman Catholic Doctrine of the Immaculate Conception also recognizes that Mary was in need of salvation, viewing her as prevented from falling into the scar of sin, instead of being pulled up out of it. Orthodox thought does vary on whether Mary actually ever sinned, though there is general agreement that she was cleansed from sin at the Annunciation.

...
Wikipedia