Solemnity of the Immaculate Conception
Mary gave to the world the Life that renews all things, and she was enriched by God with gifts appropriate to such a role.
It is no wonder, then, that the usage prevailed among the holy Fathers whereby they called the mother of God entirely holy and free from all stain of sin, fashioned by the Holy Spirit into a kind of new substance and new creature. Adorned from the first instant of her conception with the splendors of an entirely unique holiness, the Virgin of Nazareth is, on God's command, greeted by an angel messenger as "full of grace" (cf. Luke 1:28). To the heavenly messenger she replies: "Behold the handmaid of the Lord, be it done to me according to thy word" (Luke 1:38)
(Dogmatic Constitution on the Church, 56).