24 December -- O Virgo Virginum

A "bonus" antiphon.  O Virgo Virginum was sung in the Sarum Rite and in some of the continental rites, possibly the Præmonstratensian usage also.  In those usages it didn't really occur on the 24th of December but the 23d and previous antiphons were each moved back a day, beginning with O Sapientia on the 16th.

Fr East didn't post a commentary on O Virgo Virginum but someone else at that site gave a bit of information here.

More on the use of the O Virgo Virginum text in the Ordinariate usage (a.k.a. "Anglican Use") can be found here.  Indeed there is a good deal more about the expanded usage of all the Magnificat texts at that link.