O Sapientia, the name given to this day in the Calendar, denotes the first of the seven days leading up to the Christmas Vigil when one of the “O Antiphons” was to be sung at Vespers.
The O Antiphons (also known as the Antiphonae majores, the Major Antiphons) are a collection of antiphons sung in the Latin (Western) Rite with the Magnificat at Vespers for the seven days leading up to the Christmas Vigil. The origin of the antiphons is not known, though the philosopher Boethius made a slight reference to something like them in the early sixth century. There were known in the Roman Rite as early as the eighth century and there is an English poem based on them by Cynewulf (fl. eighth century). The Advent hymn Veni, veni Emanuel (“O come, O come, Emmanuel”) is a metrical text based on the antiphons that dates from the early 18th century and that was translated into English by John Mason Neale (among others) in the 19th century.
The Roman Rite made provision for seven antiphons, with one each sung with the Magnificat at Vespers on December 17 through December 23. The Sarum Use (one of the pre-Reformation forms of the Latin Rite in England) added an eighth antiphon to the original seven, O virgo virginum to the Blessed Virgin Mary. Thus the Calendar of the 1662 Book of Common Prayer designates December 16 as O Sapientia. 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 suitable for the reformed Church of England.
Each of the seven original antiphons is addressed to Christ under one or more of his scriptural titles: O Sapientia (O Wisdom), O Adonaï (O LORD), O Radix Jesse (O Root of Jesse), O Clavis David (O Key of David), O Oriens (O Dayspring), O Rex Gentium (O King of the nations), O Emmanuel.
Ignoring the initial “O”, the first letters of each antiphon spell out in Latin the reverse acrostic, SARCORE – ero cras, “I shall be [with you] tomorrow” – the last of the seven antiphons is sung at Vespers on December 23. The New Oxford Book of Carols calls the antiphon acrostic “a hidden counterpart of the joyful iteration of ‘cras’ [‘tomorrow’], which rings like a bell through the liturgy of the last week of Advent.”
Magnificat antiphon for December 17, at the evening Office
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, who proceeds from the mouth of the Most High, reaching out mightily from end to end, and sweetly arranging all things: Come to teach us the way of prudence.
My soul proclaims the greatness of the Lord,
my spirit rejoices in God my Savior;*
for he has looked with favor on his lowly servant.
From this day all generations will call me blessed:*
the Almighty has done great things for me,
and holy is his Name.
He has mercy on those who fear him*
in every generation.
He has shown the strength of his arm,*
he has scattered the proud in their conceit.
He has cast down the mighty from their thrones,*
and has lifted up the lowly.
He has filled the hungry with good things,*
and the rich he has sent away empty.
He has come to the help of his servant Israel,*
for he has remembered his promise of mercy,
The promise he made to our fathers,*
to Abraham and his children for ever.
Glory to the Father, and to the Son, and to the Holy Spirit:*
as it was in the beginning, is now, and will be for ever. Amen.
Ant. O Wisdom, who proceeds from the mouth of the Most High, reaching out mightily from end to end, and sweetly arranging all things: Come to teach us the way of prudence.
(English translations of the Latin originally taken from the website of the Order of Preachers, the Dominicans.)