In the Roman Rite, when a higher-ranked liturgical celebration impedes the celebration of a lesser one that, either permanently or (in a particular year) by coincidence, falls on the same day, the prayer of the lower-ranked celebration is usually added to that of the higher. This additional prayer is referred to as a commemoration of the lesser celebration.
In the post-Tridentine usage, on Sundays lacking the commemoration of a feast of Double rank, or of an Octave, a second and a third prayer was added to that of the Sunday. These were called "seasonal" (in Latin, pro diversitate temporum) prayers, not "commemorations". For instance, from Advent to 2 February, the first of these additional prayers was in honour of Saint Mary and the other was either that "Against the persecutors of the Church" or that "For the Pope".
At Mass, commemorations were made by adding the Collect, Secret, and Postcommunion of the commemorated celebration after the ones for the higher-ranking celebration. If three or more commemorations were made, the conclusion ("Per Dominum..." or its variants) was omitted in all but the first and last. In the Office, commemorations were made only at Vespers and Lauds, using the Antiphon on the Magnificat or Benedictus, the Versicle and Response normally following the hymn, and the collect, all said after the collect of the current day. Also at Matins, the ninth Lesson was usually read of the commemorated day, except on Sundays and on certain high-ranking feasts.
In his 1960 Code of Rubrics Pope John XXIII limited the number of commemorations allowed to two. The seasonal prayers were suppressed entirely; additionally, commemorations were divided into two classes: privileged (i.e., Sundays; I class feasts; days within the octave of Christmas; the Ember Days of September; ferias of Advent, Lent, and Passiontide; the Greater Litanies) and ordinary. Privileged commemorations were made at Lauds, Vespers, and all Masses, while ordinary commemorations were made only at Lauds, conventual Masses, and low Masses. Additional rubrics specified the number of commemorations permitted on a given day depending upon its liturgical rank (e.g., ordinary Sundays [of the II class] could incorporate privileged commemorations and ordinary commemorations of the II class; should a II class Sunday be superseded by a I or II class feast of the Lord, the Sunday was not commemorated).
The 1970 Mass of Paul VI eliminated commemorations, laying down that only one prayer should be used, admitting some flexibility in the choice of the prayer. Commemorations still exist in the Office (Liturgy of the Hours): Memorials of saints celebrated in Lent, from 17–24 December, or in the Octave of Christmas (which are all Optional Memorials) can be commemorated in the Office of Readings by adding the hagiographic reading as a third reading, concluding with the memorial's collect. Likewise, in Lauds and Vespers, similar to the Extraordinary Form, the Benedictus or Magnificat antiphon of the memorial and the collect can be said after the collect of the day.
- General Instruction to the Liturgy of the Hours, 237-239