The Holy Rule
How Many Psalms Are to Be Said at the Night Office
During the winter season, having in the first place said the verse: Deus, in adjutorium meum intende; Domine, ad adjuvandum me festina, there is next to be said three times, Domine, labia mea aperies, et os meum annuntiabit laudem tuam (Ps 50:17). To this the third psalm and the Gloria are to be added. After this the 94th psalm with its antiphon is to be said or chanted. Hereupon let a hymn follow, and after that six psalms with antiphons. When these and the verse have been said, let the Abbot give the blessing. All being seated on the benches, let three lessons be read alternately by the brethren from the book on the reading stand, between which let three responsories be said. Let two of the responsories be said without the Gloria, but after the third lesson, let him who is chanting say the Gloria. When the cantor beginneth to sing it, let all rise at once from their seats in honor and reverence of the Blessed Trinity.
Let the inspired books of both the Old and the New Testaments be read at the night offices, as also the expositions of them which have been made by the most eminent orthodox and Catholic Fathers.
After these three lessons with their responsories, let six other psalms follow, to be sung with Alleluia. After these let the lessons from the Apostle follow, to be said by heart, then the verse, the invocation of the litany, that is, Kyrie eleison. And thus let the night office come to an end.