Catholic Encyclopedia (1913)/Commemoration (in Liturgy)
The recital of a part of the Office or Mass assigned to a certain feast or day when the whole cannot be said. When two Offices fall on the same day and when, according to the rules of the rebrics, one of them cannot be transferred to another day, it is in part celebrated by way of a commemoration. Offices have different degrees of importance (doubles, semi-doubles, etc.) assigned them at their institution, and it is this that mainly determines precedence in cases of conflict.
At Mass a commemoration consists in saying the collect, Secret, and Post-Communion proper to the feast or day which is being commemorated. In the Office commemorations occur at Lauds and Vespers and consist in reciting the antiphons, with their versicles and responses, of the Benedictus and Magnificat respectively, adding in each case an oremus with the oration proper. These are called special commemorations as distinguished from the common, which are in the Office when the latter is of an inferior rite. These commemorative prayers of the Mass vary according to the season of the year. When two or more special commemorations have to be made, the order is determined by the rank or relative importance of the feasts and Offices. When two Offices fall on the same day there is said to be "occurrence"; and when the second Vespers of a preceding Office coincides with the first Vespers of the following there is "concurrence." When one of the two occurring, or concurring, Offices is very solemn and the other relatively unimportant, all mention of the latter is omitted.