How does the priest select which eucharistic prayer they will use at mass?
The priest is always free to choose which Eucharistic Prayer he wants to use. His choice is guided (but not dictated) by the GIRM:
- 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.