Starting on Dec. 17, the Church uses seven special antiphons in her liturgy. Called the "O" Antiphons (because each one starts out with "O"), they were written in Latin by an unknown author during the Middle Ages. Each one of them uses a special title for Christ and expresses the Church's longing for a Savior.
O Wisdom (sapientia in Latin)
O Lord (Adonai)
O Root of Jesse (radix)
O Key of David (clavis)
O Radiant Dawn (orient)
O King (rex)
O Emmanuel (Emmanuel)
The first letter of the Latin terms forms SARCORE, and read backward, it reads "ero cras," which in Latin means "tomorrow I will be!"
I'll post the full text of the expanded antiphon each day, because the current ones are a bit abbreviated.
Did you notice that our Advent hymn "O Come, O Come Emmanuel" is actually based on these antiphons?