This feast day celebrates the Annunciation of the Angel to Mary and the Incarnation. This feast takes place in a symbolic way 9 months before the celebration of Christmas.
It is the Angel Gabriel who announces to Mary that God had chosen her to carry out His plan of salvation.
The story of the Annunciation is found in St. Luke's Gospel (Chapter 1, Verses 26-38). Mary, by the Holy Spirit, conceived in her womb the Son of the Eternal Father. The Church has always believed, and believes, that the Virgin Mary is the "Ever Virgin"; she was Virgin before the birth, during the birth, and after the birth.
Mary accepted God's plan, pronouncing her Fiat, her yes. Her yes reflected the "yes" that Christ pronounced when He came into this world. The letter to the Hebrews speaks of this when - quoting psalm 39 - it says, "Here I am, O God, to do your will" (Hebrews 10:7). The encounter of these two "yeses'" meant that God took on a human face.
The Feast of the Annunciation is also a Christological feast because it celebrates the central mystery of Christ: His Incarnation. God desires that - in the Church - each baptized person continue to pronounce his own "fiat". That is, that each member of the Church opens himself and takes on God's will in his life, cooperating in this way His plan of salvation. We learn all of this when we look at Mary, who did not doubt the words and promises of God. On the contrary, she was faithful even at the Cross when she pronounced her "fiat" anew. "With her total obedience to God's will, Mary is ready to live everything that the divine love had foreseen for her, even the "sword" which would pierce her soul" (John Paul II, General Audience, September 4, 1996).