The hymn is of course a metrical adaptation of (five of) the Great O Antiphons; the Magnificat antiphons for Vespers for the week preceding Christmas.
The Great O Antiphons start tomorrow, which is why I thought today a good day to post on them.
I was going to blog about both the O Antiphons and the hymn, but find that the Wikipedia entries on both cover all that I was going to say, and indeed more than I knew.
The entry on the Great O Antiphons is here; there is only one thing I would argue with, and that is the suggestion that the reverse acrostic, Ero Cras, is a coincidence. There is no place for coincidence in my theology...
The entry on O Come, O Come Emmanuel is here. I was fascinated to read Neale's original version; I had only known the Ancient and Modern version.
The O Antiphons (in German) have been set wonderfully by Arvo Pärt: