Today, Dec. 8th, is the feast of the Immaculate Conception. We celebrate the fact that God created Mary to be Theotokos (God-bearer). She is, as the Angel pronounced, "Full of Grace." Because she was to be the mother of Jesus and carry him in her womb, she was born free from the effects of original sin. God gave her sanctifying grace before sin could take effect on her soul.
(edited to add an answer to a commenter's great question)