In the Latin liturgical rites of the Catholic Church, a commemoration is the recital, within the Liturgy of the Hours or the Mass of one celebration, of part of another celebration that is generally of lower rank and impeded because of a coincidence of date. Parts used in commemorating The parts commemorated are readings, antiphons, and prayers. In the Liturgy of the Hours, all three are or have been used: a reading of the commemorated celebration in Matins (Office of Readings); the antiphons of the Benedictus in Lauds and of the Magnificat in Vespers; and the proper prayer of the celebration being commemorated, the same as the collect of its Mass. In Mass, the prayers used are the collect, the prayer over the offerings and the prayer after Communion. Furthermore, before the decree Cum nostra hac aetate of 1955, in the Liturgy of the Hours the verse of the short responsory in Prime and the doxology of hymns of a commemorated feast that had special ("proper") forms of these were used, as in Mass were the commemorated feast's preface, if "proper", and the Credo, if the commemorated feast required its recitation. History Originally there were no commemorations in Mass. The older sacramentaries have only one collect. Even when, in the ninth century, priests began elsewhere to say more than one collect, only one was used in Rome. However, even in Rome the number of collects gradually increased. After the Council of Trent Pope Pius V promulgated official editions of the Roman Breviary in 1568 and the Roman Missal in 1570 pursuant to the request of the Council of Trent. These admitted of several commemorations on the same day. Thus, on 29 December the liturgy celebrated was that of Saint Thomas Becket with commemorations of the Octaves of Christmas, Saint Stephen, Saint John the Apostle, and the Holy Innocents. (See Tridentine Calendar.) Complicated rules governed such commemorations. The section De Commemorationibus in the Rubricae generales Missalis in later editions of the Missal of Pope Pius V begins by stating that "Commemorations occur at Mass as in the Office. A Double or Semidouble Feast commemorated as a Simple in the Office is commemorated also at Mass, including Solemn Mass on Class II Double Feasts, but excluding Palm Sunday and the Vigil of Pentecost. A Simple Feast is commemorated at Mass, if in the Office it was commemorated in First Vespers; but if