General Instruction of the Roman Missal: Chapter VII, The Choice of the Mass and Its Parts
II. The Choice of Mass Texts
356. In the choice of texts for the several parts of the Mass, whether of the Season or of the Saints, the following norms should be observed.
The Readings
357. For Sundays and solemnities, three readings are assigned: that is, from a Prophet, an Apostle, and a Gospel. By these the Christian people are brought to know the continuity of the work of salvation according to God’s wonderful plan. These readings should be followed strictly. During the Easter Season, according to the tradition of the Church, instead of the reading from the Old Testament, the reading is taken from the Acts of the Apostles.
For feasts, on the other hand, two readings are assigned. If, however, according to the norms a feast is raised to the rank of a solemnity, a third reading is added, taken from the Common.
For memorials of Saints, unless strictly proper readings are given, the readings assigned for the weekday are customarily used. In certain cases, readings are provided that highlight some particular aspect of the spiritual life or activity of the Saint. The use of such readings is not to be insisted upon, unless a pastoral reason suggests it.
358. In the Lectionary for weekdays, readings are provided for each day of every week throughout the entire year; as a result, these readings are for the most part to be used on the days to which they are assigned, unless there occurs a solemnity, feast, or memorial that has its own proper New Testament readings, that is to say, readings in which mention is made of the Saint being celebrated.
If, however, the continuous reading during the week is interrupted by the occurrence of some solemnity or feast, or some particular celebration, then the priest, taking into consideration the entire week’s scheme of readings, is allowed either to combine parts omitted with other readings or to decide which readings are to be preferred over others.
In Masses with special groups, the priest is allowed to choose texts more suited to the particular celebration, provided they are taken from the texts of an approved lectionary.
359. In addition, the Lectionary has a special selection of texts from Sacred Scripture for Ritual Masses into which certain Sacraments or Sacramentals are incorporated, or for Masses that are celebrated for certain needs.
Selections of readings of this kind have been established in this way, so that through a more apt hearing of the word of God the faithful may be led to a fuller understanding of the mystery in which they are participating and may be brought to a more ardent love of the word of God.
As a result, texts spoken in the celebration are to be chosen keeping in mind both a suitable pastoral reason and the options allowed in this matter.
360. At times, a longer and shorter form of the same text is given. In choosing between these two forms, a pastoral criterion must be kept in mind. At such times, attention should be paid to the capacity of the faithful to listen with understanding to a reading of greater or lesser length, and to their capacity to hear a more complete text, which is then explained in the homily.[142]
361. When a choice is allowed between alternative texts, whether they are fixed or optional, attention must be paid to what is in the best interests of those taking part, whether it is a matter of using the easier text or one more appropriate in a given group, or of repeating or setting aside a text that is assigned as proper to some particular celebration while being optional for another,[143] as pastoral advantage may suggest.
Such a situation may arise when the same text would have to be read again within a few days, as, for example, on a Sunday and on a following weekday, or when it is feared that a certain text might create some difficulties for a particular group of the Christian faithful. Care should, however, be taken that, when choosing scriptural passages, parts of Sacred Scripture are not permanently excluded.
362. The adaptations to the Ordo Lectionum Missae as contained in the Lectionary for Mass for use in the dioceses of the United States of America should be carefully observed.
The Orations
363. In any Mass the orations proper to that Mass are used, unless otherwise noted.
On memorials of Saints, 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.
On the weekdays in Ordinary Time, however, besides the orations from the previous Sunday, orations from another Sunday in Ordinary Time may be used, or one of the prayers for various needs provided in the Missal. It is always permissible, however, to use the collect alone from these Masses.
In this way a richer collection of texts is available, by which the prayer life of the faithful is more abundantly nourished.
During the more important seasons of the year, however, the proper seasonal orations appointed for each weekday in the Missal already make provision for this.
The Eucharistic Prayer
364. The purpose of the many prefaces that enrich The Roman Missal is to bring out more fully the motives for thanksgiving within the Eucharistic Prayer and to set out more clearly the different facets of the mystery of salvation.
365. The choice among the Eucharistic Prayers found in the Order of Mass is suitably guided by the following norms:
Eucharistic Prayer I, that is, the Roman Canon, which may always be used, is especially suited to be sung or said on days when there is a proper text for the Communicantes (In union with the whole Church) or in Masses endowed with a proper form of the Hanc igitur (Father, accept this offering) and also in the celebrations of the Apostles and of the Saints mentioned in the Prayer itself; it is likewise especially appropriate for Sundays, unless for pastoral considerations Eucharistic Prayer III is preferred.
Eucharistic Prayer II, on account of its particular features, is more appropriately used on weekdays or in special circumstances. Although it has been provided with its own Preface, it may also be used with other Prefaces, especially those that summarize the mystery of salvation, such as the common Prefaces. When Mass is celebrated for a particular dead person, the special formula may be inserted in the place indicated, namely, before the Memento etiam (Remember our brothers and sisters).
Eucharistic Prayer III may be said with any Preface. Its use is preferred on Sundays and feast days. If, however, this Eucharistic Prayer is used in Masses for the Dead, the special formula for the dead may be used, to be included at the proper place, namely, after the Omnes filios tuos ubique dispersos, tibi, clemens Pater, miseratus coniunge (In mercy and love unite all your children).
Eucharistic Prayer IV has an invariable Preface and gives a fuller summary of salvation history. It may be used when a Mass has no Preface of its own and on Sundays in Ordinary Time. Because of its structure, no special formula for the dead may be inserted into this prayer.
The Chants
366. It is not permitted to substitute other chants for those found in the Order of Mass, such as at the Agnus Dei.
367. The norms laid down in their proper places are to be observed for the choice of the chants between the readings, as well as of the chants at the entrance, at the offertory, and at Communion (cf. nos. 40-41, 47-48, 61-64, 74, 86-88).