The O Antiphons of Advent

Today is the final day of the O Antiphons.

"In the Roman Catholic tradition in which they originated, the O Antiphons are sung or recited at Vespers from December 17 to December 23 inclusive. In the Church of England they have traditionally been used as antiphons to the Magnificat at Evening Prayer during this period, and although not printed in the Book of Common Prayer, have long been part of secondary Anglican liturgical sources, such as the English Hymnal. More recently they have found a place in primary liturgical documents throughout the Anglican Communion, including the Church of England's Common Worship liturgy." Wikipedia

 The hymn O come, O come, Emmanuel (in Latin, Veni Emmanuel) is a lyrical paraphrase of these antiphons.