March 25, which is nine months before the feast of the Nativity of Jesus Christ, or Christmas, is the traditional date for the commemoration of the Annunciation of the Theotokos and Mary, the mother of Jesus. This was also the New Year in many places, including England and the American colonies.
According to the gospel of Luke, 1:26 ff, The Archangel Gabriel was sent to Nazareth in Galilee, to the virgin Mary, and Announced to her that she had found favor with God and would bear a son, Jesus.
She asked how that would be, since although she was pledged to be married to Joseph the Betrothed, she was pledged to remain a virgin. The angel replied that she would conceive through the Holy Spirit. Mary consented, saying "Behold the handmaid of the Lord; be it unto me according to thy word." Hence Mary became the Theo-tokos, the God-bearer.
This is a traditional Eastern Orthodox hymn for this day:
Today is the beginning of our salvation, The revelation of the eternal mystery! The Son of God becomes the Son of the Virgin As Gabriel announces the coming of Grace. Together with him let us cry to the Theotokos: Rejoice, O Full of Grace, The Lord is with You!