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 prayer going into evening and when the O Antiphons are prayed or sung.
Going back to our original article, it says:
The importance of “O Antiphons” is twofold: Each one highlights a title for the Messiah: 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 Rising Sun), O Rex Gentium (O King of the Nations), and O Emmanuel. Also, each one refers to the prophecy of Isaiah of the coming of the Messiah.
O Adonai – O Lord
O Radix Jesse – O Root of Jesse
O Clavis David – O Key of David
O Oriens – O Rising Sun
O Rex Gentium – O King of Nations
O Emmanuel – O God with Us
Our first O Antiphon is O Sapientia, O Wisdom. Today, as we begin our O Antiphons, let us pray for, sing for and look for the wisdom that is to come in the form of a baby, a tiny child who will come to save us.