The feast of the Annunciation of the Lord
The feast of the Annunciation of the Lord celebrates the angel Gabriel's appearance to the Virgin Mary (Luke 1:26-38), his announcement that the Blessed Virgin had been chosen to be the mother of Our Lord, and Mary's fiat—her willing acceptance of God's holy plan.
HISTORY OF THE ANNUNCIATION OF THE LORD
Originally a feast of our Lord, but now celebrated as a Marian feast, the feast of the Annunciation dates back at least to the fifth century, and the date of the feast, which is determined by the date of Christmas, was set at March 25 by the seventh century.
The Annunciation, as much as or even more so than Christmas, represents Christ's Incarnation. When Mary signaled to Gabriel her acceptance of God's Will, Christ was conceived in her womb through the power of the Holy Spirit. While most of the fathers of the Church say that Mary's fiat was essential to God's plan of salvation, God foresaw Mary's acceptance of His Will from all eternity.
The narrative of the Annunciation testifies powerfully to the truth of the Catholic tradition that Mary was indeed a virgin when Christ was conceived, but also that she intended to remain one perpetually. Mary's response to Gabriel—"How shall this be done, because I know not man?" (Luke 1:34) was universally interpreted by the fathers of the Church as a statement of Mary's resolution to remain a virgin forever.