On Dec. 17, we enter the octave leading up to Christmas. The daily office takes on a more anticipatory tone. One highlight of this period are the "O Antiphons," which are the antiphons preceding the Magnificat, that is prayed during Vespers (evening prayer) during this week.
What are the O Antiphons? (Catholic Education article)
More at the Anchoress.
Love to be Catholic has a video up with the chanted antiphon for each day.
Another one for Dec. 18 (O Adonai), a YouTube video of the Domonican brothers singing the antiphon, at ID, and another one for Dec. 17. (O Sapientia).