December 17th marks the beginning of the O Antiphons.
From the linked article:
The “O Antiphons” refer to the seven antiphons that are recited (or chanted) preceding the Magnificat during Vespers of the Liturgy of the Hours. They cover the special period of Advent preparation known as the Octave before Christmas, Dec. 17-23, with Dec. 24 being Christmas Eve and Vespers for that evening being for the Christmas Vigil.
The Liturgy of the Hours is the prayer of of the church as we literally “pray through the day.” Many of us in the secular life use an abbreviated form of this prayer, but in monasteries, convents and all sorts of places, this is prayed daily.
Vespers is the Continue reading