O Wisdom, which camest out of the mouth of the most High, and reachest from one end to another, mightily and sweetly ordering all things: Come and teach us the way of prudence.
Starting tonight, Anglican monastics, and others, will sing the Great O Antiphons as the Antiphons upon the Magnificat at Vespers, for a period of eight days.
Roman Catholics start one day later, because they use only 7 of the Antiphons; ironically, the one the Romans don't use - and Anglicans do - is a Marian antiphon, the last one on the Anglican calendar, "O Virgo Virginum," sung on December 23rd. OVV is Sarum in origin; more about that here, from which the following is an excerpt:
The Advent Antiphons in preparation for Christmas, based on Old Testament prophecies of the Messiah under various titles and figures, are found in eleventh century manuscripts. But they must be of much earlier origin; for Amalarius, a French liturgical scholar of the first half of the ninth century, added an eighth to the older seven. This, O Virgo virginum, is not on parallel lines with the others, nor is it found in the Roman Breviary, but it had place in the Sarum.
The O's are not found at all in the Ambrosian Breviary, which has an Advent of six weeks, the last Sunday being a commemoration of the Annunciation.
The Parisian Breviary (1735), of which a marked characteristic is the use of Holy Scripture in antiphons and responds, adds two to the original seven Advent Antiphons, of like nature with them, O Sancte Sanctorum, and O Pastor Israel. Thus provision is made for a complete Novena, from December 15 to 23, before Christmas Eve.
The Roman Breviary does not begin the Antiphons till December 17, but December 16 is the English date (marked in the Prayer Book Kalendar as O Sapientia), either St. Thomas's Day being otherwise provided for, or O Virgo virginum being added as an eighth.
The Antiphons were sung at Vespers before and after the Magnificat.
Here's a really excellent new piece that includes Biblical sources and mp3s of each. Here's a good article that includes a score of the music; here's another.
There are some new audio files, too. Here's one sung by a guy with a really soft, warm baritone (or maybe bass) voice. There is also a version at the link above, and another one here, sung along with the Magnificat.