IV Sunday of Advent vacat: it is neither celebrated nor commemorated; the Vigil of the Nativity takes precedence.
Season of the Nativity begins with I Vespers.
Mass is of the Vigil of the Nativity, not of the IV Sunday of Advent.
Proper Mass; no Gloria; after Gradual, say Alleluia and following verse; Credo; Preface of the Holy Trinit; no commemoration
Matins: Invitatorium Hodie; Hymn Verbum supernum; 9 antiphons and 9 psalms of Sunday, with verse Hodie; 3 lessons with their responsories of the Vigil; no Te Deum
Lauds: Proper antiphons and Sunday psalms of the 1st scheme; Hymn En clara vox; no commemoration of the 4th Sunday of Advent
Hours: Sunday psalms with antiphons from Lauds; Prime as on feasts and psalm 53 is said in place of psalm 117
I Vespers: Of the Nativity of the Lord; Hymn Iesu, Redemptor omnium
Compline: Of Sunday. Nativity versicle (Post partum) after Marian antiphon, until February 1.