History:
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.
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.
By Scott P. Richert, About.com Guide