In the calendar of the English prayer book of 1662 December 16 has this cryptic entry. It is the opening two words for the antiphon for the Magnificat in the Sarum and other pre-Reformation orders of vespers. In the Roman rite this antiphon is found on 17 December but the sequence of antiphons follows the same order as that in the Sarum office with the exception that Sarum had an additional "Great O" not found in the continental rites. What most Anglicans don't know is that this antiphon and those following it go back to the fifth century. We know this because Boethius (480 - 524/5) makes a passing reference to same as if everyone reading him would understand the allusion. The question for prayer book Anglicans is why when so much of the ancient rites were suppressed that the calendar should contain this quite unexplained reference?
It is almost as if there were a secret wish for it to be revived and with the Oxford Movement and its aftermath this has occurred in the English Church. It began with the monastic revival and books like The Day Hours of the Church of England. These books were essentially translations of the Sarum office book which had received these antiphons as part of the common heritage of the Western Church. Further when the Rev'd G. H. Palmer translated the Sarum Diurnal for the use of the Community of St Mary the Virgin at Wantage, he also set the antiphons to their ancient music so even though the words were now in English the tune would be the same. Consequently for the days that the Great O's were sung, the Magnificat would be sung to the solemn version of tone II with the second ending. This is one of the most beautiful of the solemn tones which anciently were always used for both the Magnificat and the Benedictus. They are, as one might imagine, slightly more elaborate than the simple versions of the same tone.
There is another interesting thing about the Great O's. If one uses only the Roman version of them. reading backwards from the last to the first, a Latin phrase is formed, "ero cras" - tomorrow I will come. This is the essential promise of the Advent season and the thread that is woven from all of the biblical texts referenced in the words of the antiphons themselves. In the case of O Sapientia the following verses are evoked in the text: Isa. 11. 2,3; Isa. 28.29; Sirach 24.3; and Wisdom 8.1.
O Sapientia, quae ex ore Altissimi prodiisti,
attingens a fine usque ad finem,
fortiter suaviterque disponens omnia:
veni ad docendum nos viam prudentiae.
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.