Today is the Solemnity of the Annunciation of the Lord; this day was chosen for the feast because it is nine months before the Solemnity of Christmas. Today's feast celebrates the coming of the Angel Gabriel to the Virgin Mary to announce to her the special mission God had chosen for her in being the mother of His only son. We are continually reminded of the importance of this feast to our salvation in various devotional prayers. For example, in the joyous mysteries of the Rosary, this feast is highlighted, as it is in the Angelus.
The feast of the Annunciation began to be celebrated on this day during the fourth and fifth centuries, soon after the date for celebrating Christmas was universalized throughout the Church. This feast celebrates the Incarnation of the Second Person of the Trinity and the salvation of all mankind. This point of our salvation was deeply discussed by many of the Church fathers, to explain it to the faithful and to show the deep love God has for us. Some of the Church fathers who wrote on this were St. Athanasius, St. Gregory of Nyssa and St. Augustine.
Enriched from the first instant of her conception with the splendor of an entirely unique holiness, the Virgin of Nazareth is hailed by the heralding angel, by divine command, as 'full of grace' (cf. Lk 1:28). To the heavenly messenger she replies: 'Behold the handmaid of the Lord; be it done to me according to thy word' (Lk 1:38). Thus the daughter of Adam, Mary, consented to the word of God, became the Mother of Jesus. Committing herself wholeheartedly and impeded by no sin to the Lord, to the person and work of her Son, under and with him, serving the mystery of redemption, by the grace of Almighty God. -Constitution on the Church, 56