In the final days before the celebration of Christmas (December 17-23), the Church proscribes the reciting or singing of the O Antiphons. These verses each contain a title of Jesus as found in Old Testament prophecy.
However, this tradition is sometimes missed, especially in the busyness of a lay person's life.
Traditionally these antiphons are recited or sung during Vespers (Evening Prayer). The antiphon is found immediately before the recitation of the Magnificat. Anyone who has a copy of the Liturgy of the Hours can participate in this progression each day.
Dom Prosper Guéranger explains why these antiphons are situated in the context of Vespers in his Liturgical Year:
The canonical Hour of Vespers has been selected as the most appropriate time for this solemn supplication to our Savior, because as the Church sings in one of her hymns, it was in the Evening of the world (vergente mundi vespere) that the Messias came amongst us. These Antiphons are sung at the Magnificat to show us that the Saviour whom we expect is to come to us by Mary.
Not everyone has a copy of the Liturgy of the Hours, and so the Church has included them as a Gospel acclamation on these days during daily Mass.
Furthermore, not everyone attends daily Mass, so the most popular Advent hymn, O Come, O Come Emmanuel, features each of these O Antiphons and is frequently used during Sunday Masses in Advent.
