The recital of a part of the office of a certain feast or day when the entire office cannot be said because it is impeded by one of higher rank.
Commemorations are made at Vespers and Lauds and consist of antiphon, versicle, and prayer; they are sometimes made at Matins by means of the ninth lesson.
A commemoration of the Holy Cross is made during Paschal time.
At Mass the Commemoration is made by adding Collect, Secret, and Postcommunion of the feast or saint commemorated.
New Catholic Dictionary