Like I said, this is one of my favorite Christmas hymns, and the official Latin edition of the modern Roman breviary gives it as the hymn for first and second vespers of the feast of Mary, the Holy Mother of God. (Arguably the oldest Marian feast of the kalendar, and certainly one of the most profound and beautiful of the church's year.)
I'm still using my vesperal, only now, it's onto Christmastide. All of this office is taken from the older liturgical books, including the rather tricky and yet, quite beautiful (both musically and textually) antiphon "O admirabile Commercium", and the magnificat antiphon "Propter nimiam caritatem suam". The latter I've sung in my head so many times, that I have no problem with it.
I'm using it as a text for my meditations as I prepare for my total consecration on the feast day. It's been 33 days, and I hope I've done it right. If not, a short day of devotion tomorrow will hopefully make up for it!