The Annunciation (or Evangelismos in Greek) to the Theotokos is one of the Great Feasts of the Orthodox Church, celebrated on March 25. Greeks also celebrate Greek Independence Day this day. This is one of only two days during Lent, the other being Palm Sunday, when fish is permitted.
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 (or vice-versa?), indicating that Christ was conceived in perfection at that time "of the Holy Spirit and the Virgin Mary," as stated in the Nicene-Constantinopolitan Creed.
Luke 1:26-38 New King James Version (NKJV)
Christ’s Birth Announced to Mary26 Now in the sixth month the angel Gabriel was sent by God to a city of Galilee named Nazareth, 27 to a virgin betrothed to a man whose name was Joseph, of the house of David. The virgin’s name was Mary. 28 And having come in, the angel said to her, “Rejoice, highly favored one, the Lord is with you; blessed are you among women!”[a]
29 But when she saw him,[b] she was troubled at his saying, and considered what manner of greeting this was. 30 Then the angel said to her, “Do not be afraid, Mary, for you have found favor with God. 31 And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS. 32 He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His father David. 33 And He will reign over the house of Jacob forever, and of His kingdom there will be no end.”
34 Then Mary said to the angel, “How can this be, since I do not know a man?”
35 And the angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God. 36 Now indeed, Elizabeth your relative has also conceived a son in her old age; and this is now the sixth month for her who was called barren. 37 For with God nothing will be impossible.”
38 Then Mary said, “Behold the maidservant of the Lord! Let it be to me according to your word.” And the angel departed from her.