Q: How do presiders decide which eucharistic prayer to use?
A: There is no formal rule. Eucharistic Prayer I is especially appropriate for Christmas, Epiphany, Easter, Ascension of the Lord and Pentecost because it has variable parts related to them. Also, it is recommended on the feasts of saints mentioned there.
Some eucharistic prayers, especially the two for Reconciliation and the four for special occasions, may particularly reinforce a day’s readings or be especially appropriate for that day’s feast.