One of the best known Advent hymns is O come, O come, Emmanuel which is derived from the Great Advent Antiphons.  Traditionally there are seven Antiphons, all beginning with ‘O’ and therefore often called the ‘O’ Antiphons.  They were sung in the last week of Advent at the service of Vespers, before and after the Magnificat.  In the Book of Common Prayer, December 16th is the date of the first Antiphon and in the Prayer Book Calendar you will see the words, ‘O Sapientia’ on that date.  In Common Worship, as in the Roman Breviary, the start date is December 17th.  The seven ‘O’s in Latin and English, in the order in which they are sung, one each day, are as follows:

O Sapientia-------O Wisdom   
O Adonai---------O Lord
O Radix Jesse----O Root of Jesse
O Clavis David----O Key of David
O Oriens----------O Morning Star
O Rex Gentium---O King of the Nations
O Emmanuel------O Emmanuel (meaning ‘God is with us’)

When viewed backward from the last Antiphon to the first, the first letters of the Latin texts (shown in red above) spell out the Latin words ero cras, meaning ‘I come tomorrow’. (4) The Antiphons and the relevant scriptural references are included in Common Worship: Advent on pages 58-59.