...From (Dec.17) until the day before Christmas Eve, at the Magnificat during vespers in the Roman rite, seven antiphons are sung, one per day, all of them beginning with an invocation to Jesus, although he is never called by name.
The antiphons are very old, going back to the time of Pope Gregory the Great, around the year 600. They are in Latin, and are inspired by the texts of the Old Testament proclaiming the Messiah.
At the beginning of each antiphon, in order, Jesus is invoked as Wisdom, Lord, Root, Key, Star, King, Emmanuel. In Latin: Sapientia, Adonai, Radix, Clavis, Oriens, Rex, Emmanuel.
Read starting from the last, the Latin initials of these words form an acrostic: "Ero cras," meaning: "I will be [there] tomorrow." It is the proclamation of the Lord who comes. The last antiphon, which completes the acrostic, is sung on December 23, and the following day, with first vespers, the feast of the Nativity begins...
(h/t First Things)