December 17 marks the beginning of the "O" Antiphons an ancient part of our liturgy, dating back to the fourth century, one for each day until Christmas Eve. These antiphons address Christ with seven magnificent Messianic titles, based on the Old Testament prophecies and types of Christ. The Church recalls the variety of the ills of man before the coming of the Redeemer.

The antiphons are part of the evening prayer of the Divine Office, the antiphon before and after the Magnificat. They are also the alleluia verse before the Gospel at Mass.

http://www.catholicculture.org/culture/ ... cfm?ID=234