Traditional day of fast and abstinence in the USA. This may be observed ad libitum on the 23rd.
The Season of the Nativity begins with I Vespers
Proper Mass; no Gloria; after Gradual, omit Alleluia and following verse; no Credo; common preface
Matins: Invitatorium Hodie scietis; Hymn Verbum supernum; 9 antiphons and 9 psalms of Sunday, with verse Hodie; 3 lessons with the responsories of the Vigil; no Te Deum
Lauds: Proper antiphons and Sunday psalms of the 1st scheme; Hymn En clara vox; remainder proper
Prime: Antiphon from Lauds, with Sunday psalms (Psalm 53 instead of 117); remainder proper
Hours: Sunday psalms with antiphons from Lauds; remainder proper
I Vespers: Of the Nativity of the Lord; Hymn Iesu, Redemptor omnium; remainder proper
Compline: Of Sunday. Nativity versicle (Post partum) after Marian antiphon, until February 1.