You’ve most likely heard the hymn “O Come, O Come, Emmanuel” played during the holiday season.
What you might not know is that it refers to a yearly tradition that begins on December 17 each year, namely, the “O Antiphons.”
It is the Christian belief that Jesus Christ was the Messiah, and that his coming was prophesied by special titles throughout the Old Testament. Seven of these titles form the “O Antiphons”—so called because they each begin with the vocative “O” (like in “O Canada”). They date back to at least the sixth century, and are used by many Christians during their evening prayer (Vespers) as a special preparation leading up to Christmas.
The titles are, in order:
Dec 17: O Wisdom
Dec 18: O Lord
Dec 19: O Root of Jesse
Dec 20: O Key of David
Dec 21: O Dayspring
Dec 22: O King of the Nations
Dec 23: O Emmanuel
Here is what the first antiphon for today (beginning with the highlighted "Wisdom") looks like in the official Roman Catholic prayer book (a “breviary”):
One other cool thing… The titles were originally proclaimed in Latin, and their acrostic, read backwards, forms the Latin phrase “ERO CRAS,” which translated means “Tomorrow, I will be (there)”—a coded reference to Christ’s birth on the day after the antiphons end.