The O Antiphons start Monday, so I thought today would be a nice day to refresh them. I notice that Wikipedia has a very nice article on the subject. An excerpt:

Each antiphon is a name of Christ, one of his attributes mentioned in Scripture. They are:

December 17: O Sapientia (O Wisdom)
December 18: O Adonai (O Lord)
December 19: O Radix Jesse (O Root of Jesse)
December 20: O Clavis David (O Key of David)
December 21: O Oriens (O Dayspring)
December 22: O Rex Gentium (O King of the nations)
December 23: O Emmanuel (O With Us is God)

In the Roman Catholic tradition in which they originated, the O Antiphons are sung or recited at Vespers (official Evening Prayer) from December 17 to December 23 inclusive.