The Annunciation
The Annunciation (or Evangelismos in Greek) to the Theotokos is one of the Great Feasts of the Orthodox Church, celebrated on March 25.
According to the Gospel of Luke 1:26-38, the Archangel Gabriel appeared to Mary to announce to her that she would conceive and bear a son, even though she "knew no man." According to holy tradition Mary had come home to her parents when she was only fifteen when she was visited by Gabriel.
This date was selected by the Church Fathers to be exactly nine months ahead of the Nativity of Our Lord, indicating that Christ was conceived in perfection at that time "of the Holy Spirit and the Virgin Mary," as stated in the Nicene Creed.