In the western church, the “O antiphons” have traditionally been sung at Vespers before and after the Magnificat, in the period from 17 December to 23 December.
However, in the calendar for the Book of Common Prayer, 16 December is marked as “O Sapientia”. Why the 16th rather than the 17th?
As this post explains, 16 December was the date the O antiphons began in the Sarum Use. In this scheme, eight antiphons (rather than seven) were used, the extra antiphon being O virgo virginum, sung on 23 December:
O Virgo virginum, quomodo fiet istud?
Quia nec primam similem visa es nec habere sequentem.
Filiae Jerusalem, quid me admiramini?
Divinum est mysterium hoc quod cernitis.
O Virgin of virgins, how shall this be?
For neither before thee was any like thee, nor shall there be after.
Daughters of Jerusalem, why marvel ye at me?
The thing which ye behold is a divine mystery.
Cranmer drew heavily on the Sarum Use in writing the Book of Common Prayer, which explains the retention of 16 December as the date for O Sapientia. Except it doesn’t, because beyond that mysterious reference in its calendar the Prayer Book makes no provision at all for the O antiphons. As the post linked above observes:
Interestingly, while the 1662 Calendar preserved the pre-Reformation English date, there is no evidence for the use of the O Antiphons in Anglican worship in the 17th century, and the Marian antiphon appointed for December 23 in Sarum Use would not have been sung in the reformed Church of England at the time.
Finally, as Wikipedia observes, the inclusion of O virgo virginum changes the acrostic formed by the initial letters of each “O” (written in reverse order) from ERO CRAS (“I shall be with you tomorrow”) to VERO CRAS (“truly tomorrow”).