General Instruction of the Roman Missal: Chapter VII, The Choice of the Mass and Its Parts
352. The pastoral effectiveness of a celebration will be greatly increased if the texts of the readings, the prayers, and the liturgical songs correspond as closely as possible to the needs, spiritual preparation, and culture of those taking part. This is achieved by appropriate use of the wide options described below.
The priest, therefore, in planning the celebration of Mass, should have in mind the common spiritual good of the people of God, rather than his own inclinations. He should, moreover, remember that the selection of different parts is to be made in agreement with those who have some role in the celebration, including the faithful, in regard to the parts that more directly pertain to each.
Since, indeed, a variety of options is provided for the different parts of the Mass, it is necessary for the deacon, the lectors, the psalmist, the cantor, the commentator, and the choir to be completely sure before the celebration about those texts for which each is responsible is to be used and that nothing be improvised. Harmonious planning and carrying out of the rites will be of great assistance in disposing the faithful to participate in the Eucharist.
I. The Choice of Mass
353. On solemnities the priest is bound to follow the calendar of the church where he is celebrating.
354. On Sundays, on the weekdays of the Advent, Christmas, Lenten, and Easter Seasons, on feasts, and on obligatory memorials:
If Mass is celebrated with a congregation, the priest should follow the calendar of the church where he is celebrating;
If Mass is celebrated with the participation of one minister only, the priest may choose either the calendar of the church or his own proper calendar.
355. On optional memorials,
On the weekdays of Advent from December 17 to December 24, on days within the Octave of Christmas, and on the weekdays of Lent, except Ash Wednesday and during Holy Week, the Mass for the current liturgical day is to be used; but the collect may be taken from a memorial which happens to be listed in the General Calendar for that day, except on Ash Wednesday and during Holy Week. On weekdays of the Easter Season, memorials of Saints may rightly be celebrated fully.
On the weekdays of Advent before December 17, the weekdays of the Christmas Season from January 2, and the weekdays of the Easter Season, it is possible to choose either the weekday Mass, or the Mass of the Saint, or the Mass of one of the Saints whose memorial is observed, or the Mass of any Saint listed in the Martyrology for that day.
On the weekdays in Ordinary Time, it is possible to choose either a weekday Mass, or the Mass of an optional memorial which happens to occur on that day, or the Mass of any Saint listed in the Martyrology for that day, or a Mass for Various Needs, or a Votive Mass.
If he celebrates with a congregation, the priest will take care not to omit the readings assigned for each day in the Lectionary for weekdays too frequently and without sufficient reason, since the Church desires that a richer portion at the table of God’s word be provided for the faithful.[141]
For the same reason he should use Masses for the Dead in moderation, since every Mass is offered for both the living and the dead, and there is a commemoration of the dead in the Eucharistic Prayer.
Where, however, the optional memorials of the Blessed Virgin Mary or of the Saints are dear to the faithful, the priest should satisfy their legitimate devotion.
When, on the other hand, the option is given of choosing between a memorial found in the General Calendar and one found in a diocesan or religious calendar, preference should be given, all things being equal and in keeping with tradition, to the memorial inscribed in the particular calendar.