I think the majority of people who sing the popular “O come, o come, Emmanuel” have no real idea of the background to this carol. Do you?
In the daily office, the regular invariability of singing/chanting/praying of psalms and biblical canticles (songs) is “softened” by an antiphon, a line usually said/sung before, and then repeated after, the psalm or canticle. So while the psalms and canticles don’t change, the antiphons colour the feast day or season.
For the next seven days, the daily praying of the Magnificat is surrounded by seven different antiphons, one each day. We have been using these (what are called) O Antiphons for at least thirteen centuries. The carol, “O come, o come, Emmanuel” brings together all seven of those antiphons.
I have a passion for these O Antiphons.
Today, 17 December, the antiphon before and after the Magnificat is:
O wisdom, coming forth from the Most High, filling all creation and reigning to the ends of the earth; come and teach us the way of truth.
This is like a biblical liqueur, distilling numerous biblical images and concepts (eg. Ecclesiasticus 24.3-9 Proverbs 1:20; 8; 9 and 1 Corinthians 1:30). Roman Catholics and others also use them in the Alleluia verse before the Gospel reading in the daily Eucharist (I guess so that those who do not pray the daily office have a chance to encounter them, and those who do pray the daily office have another chance to do so).
There is a wealth of material on these wonderful O Antiphons on this site.
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)