COMMEMORATION. – When, because of the occurrence of a feast of a higher rite, the Office and Mass of one or more saints, or of a feria, vigil, or octave, cannot be recited, a commemoration or memorial of them is often made in the Office and Mass of the day.
The cycles of the Temporal and the Sanctoral, in fact, run simultaneously, and the Martyrology records many saints who on the same day ascended to the glory of heaven. It is therefore not uncommon for a conflict to arise in the calendar between a feast, a Sunday, a feria, a vigil, or an octave. The liturgical rules seek to eliminate or lessen these conflicts, either by suppressing the less important feasts, transferring the more noble ones, or simply commemorating some of them. This commemoration consists of an antiphon with its corresponding versicle and prayer at Vespers and Lauds, and of the prayer at Mass.