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.
TheoFonsAd fontes fidei