On the Annunciation the archangel Gabriel comes to a young woman in Nazareth and announces that she will bear the Son of God. The Church counts this as the beginning of our salvation, for at her free consent, “be it unto me according to thy word”, the eternal Word takes flesh in her womb. God does not save humanity over its head; He waits for the assent of a human heart.
The Fathers see the Virgin as the bush that Moses saw burning yet not consumed: she bears the fire of the Godhead and is not destroyed. Her yes undoes the no of Eden; where the first woman's disobedience opened the door to death, this obedience opens it to life. The feast is therefore as much about human freedom as about divine mercy: heaven asks, and does not compel.
The Annunciation falls on March 25, nine months before the Nativity, and often within Great Lent, so that even in the season of repentance the Church keeps a feast of joy, the first word of the Gospel spoken into the fast.