The observance of memorials is integrated into the celebration of the occurring weekday (the "feria") in accord with the norms set forth in the General Instructions of the Roman Missal and the Liturgy of the Hours.
The General Instruction of the Roman Missal lays down that, for memorials of saints, unless proper readings are given, the readings assigned for the weekday are customarily used. In certain cases, appropriate readings are provided that truly shed light on a special aspect of the spiritual life or activity of the Saint. The use of such readings is not to be insisted upon, unless a compelling pastoral reason suggests it. The Collect proper to the day is used or, if none is available, one from an appropriate Common. The Prayer over the Offerings, however, and the Prayer after Communion, unless they are proper, may be taken either from the Common or from the weekdays of the current Season.
The General Instruction of the Liturgy of the Hours gives the following indications on celebration of memorials occurring on ordinary days: In the Office of Readings and at Lauds and Vespers:
- a) All the psalms with their antiphons are taken from the current weekday, unless the memorial has proper antiphons and psalms.
- b) If the memorial has its own antiphon for the invitatory, hymn, short reading, Benedictus and Magnificat antiphons and intercessions, these are used. Otherwise these elements are taken either from the Common or from the Office of the current week and day.
- c) The concluding prayer is taken from the Office of the saint.
- d) In the Office of Readings, the biblical reading with its responsory is that assigned to the weekday. The hagiographical second reading with its responsory is proper to the saint, but if no proper reading is assigned, the reading is either taken from the Common or is the patristic reading of the weekday
- e) Prayer during the Day and Compline are taken entirely from the weekday.
Celebrations of memorials occurring between 17 December and 24 December and during Lent, which are then never obligatory, consist of adding to the Office of Readings, after the patristic reading and responsory of the weekday, the hagiographical reading and responsory of the saint, and concluding with the prayer of the saint; and adding to Lauds and Vespers, after the concluding prayer of the weekday, the antiphon (proper or common) and the prayer of the saint.
Obligatory and optional memorials
Memorials are either obligatory or optional. The rules governing the celebration of memorials, whether obligatory or optional, are identical. The only difference is precisely that an optional memorial need not be observed, and, with the limitations indicated for the second part of Advent and for Lent, there is the possibility of celebrating instead the Mass either of another memorial assigned to that day, or of the weekday, or of any saint mentioned in the Roman Martyrology for that day, or indeed (except during the first part of Advent, the days from 2 January to the day before Epiphany, and Eastertide), a Mass for Various Needs, or a Votive Mass.
Sometimes even those memorials that are called obligatory cease to be such. This happens every year to those that happen to fall within Lent. If two obligatory memorials occur on the same day (as can happen when the movable memorial of the Immaculate Heart of Mary falls on the same date as a fixed obligatory memorial), both become optional.