Tradition
St. Benedict of Nursia
The Rule of St. Benedict
Chapter XIV. How the Night Office Is to Be Said on the Feasts of the Saints
CHAPTER XIV
How the Night Office Is to Be Said on the Feasts of the Saints
On the feasts of the saints and on all solemn festivals let the night office be performed as we said it should be done on Sunday; except that the psalms, the antiphons, and the lessons proper for that day be said; but let the number above mentioned be maintained.
Source: The Rule of St. Benedict (CCEL)