The Vigil of the Nativity is unique in the Liturgical Year in that it is of simple rite at Mattins and then becomes of double rite from Lauds onwards.
Mattins has one nocturn of three lessons. The invitatoy is Hodie scietis quia veniet Dominus, Ex mane videbitis gloriam ejus and the hymn is Verbum supernum prodiens. The antiphons Exsultate etc are sung with psalms 80, 81, 82, 83, 84, 85, 86, 87, 88, 93, 95 & 96 from the Psalter for Friday. The psalms are sung in pairs under six antiphons. After the last antiphon a versicle proper to the day, Hodie scietis, quia veniet Dominus, and its respond, Et mane videbitis gloriam ejus. The homily is taken from St. Jerome's commentary on the first chapter of St. Matthew's Gospel. The first lesson consists of the text of both the first and second lessons in the post-Clementine books. The second lesson is the same as the modern third lesson. The third lesson, Joseph fili David noli timere (...) salvum faciet populum suum, is entirely absent from the post-Clementine books. A third responsory is sung in place of the Te Deum. At Lauds the antiphons, Judaea et Jerusalem nolite timere etc (sung under double rite), are proper to the day and are sung with the psalms of Sunday (92, 99, 62-66, Benedicite and 148-149-150. The Office hymn is Vox clara ecce intonat. The V & R, chapter, antiphon on the Benedictus, and collect are proper to the Vigil. The ferial preces are not sung.
Prime is festal with the first antiphon from Lauds, as usual, and psalms 53, 118(i) and 118(ii). The Martyrology is sung with extra solemnity today. The Hebdomadarius dons a violet cope and preceded by acolytes bearing candles and a thurifer with incense enters the choir. After reverencing the choir and altar the Martyrology is censed, as a Gospel book, three times. The the choir rise and the Hebdomadarius chants 'Octavo Kalendas Januarii. Then, after announcing the moon, continues Anno a creatione mundi, quando in principio Deus creavit coelum et terram, quinquies millesimo centesimo nonagesimonono: A diluvio etc., listing the years since the birth of Abraham, the Exodus from Egypt, the anointing of David, the time since Daniel the Prophet, since the founding of Rome and the conception of the LORD by the Holy Ghost '...novemque post conceptionem decursis mensibus', then raising the pitch of the chant, whilst the choir kneel, he continues, 'in Bethlehem Judae nascitur ex Maria Virgine factus Homo'. Then in the tone of the Passion: 'Nativitas Domini nostri Jesu Christi secundem carnem.' The choir then rise and sit whilst in the normal tone the Hebdomadarius (or a lector, depending on the custom of the place) continues with the entries for the day: 'Eodem die natalis santae Anastasiae etc. The lectio brevis, Per quem accepimus, is proper to the Vigil. The antiphons from Lauds are used in sequence at the rest of the Hours.
Mass is sung after None. Today the ministers do not wear folded chasubles but dalmatic and tunicle. There is just one collect. The dismissal, Benedicamus Domino, is sung by the deacon to a most beautiful and ornate tone reserved for today, the feast of the Holy Innocents and pro re gravi Masses.
First Vespers of the Nativity are sung in the afternoon. The antiphons Rex pacificus etc are sung with psalms 109, 110, 111, 112 & 116. All hymns of Iambic metre have the Doxology Gloria tibi Domine, Qui natus es de Virgine for the Octave and up until the feast of the Epiphany.