Prayers at Mass, and regarding the options for them:
363. In any Mass the orations proper to that Mass are used, unless otherwise noted.
On Memorials of Saints, the proper Collect is said or, if this is lacking, one from an appropriate Common. As to the Prayer over the Offerings and the Prayer after Communion, unless these are proper, they may be taken either from the Common or from the weekday of the current time of year.
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. However, it shall always be permissible to use from these Masses the Collect alone.
In this way a richer collection of texts is provided, by which the prayer life of the faithful is more abundantly nourished.
However, during the more important times of the year, provision has already been made for this by means of the orations proper to these times of the year that exist for each weekday in the Missal.
The prayer life of the faithful would be even more abundantly nourished if these prayers were rendered in good, comprehensible English. But in theory, the principle here is a good one.