Question: How is the class of a saint's day determined? What are "memorials" and "optional memorials"? (P.L. GA)
Answer: Most of the saints' days fall on a particular date in the calendar year, and are assigned a rank based on the importance of the person celebrated. Feasts of our Lord are always of the first or second class (e.g. Christmas, Transfiguration); those honoring the life of our Lady are usually first or second class (e.g. Immaculate Conception, Visitation), and those attributing some title to her are usually second or third class (e.g. Seven Sorrows, Consolation); the Apostles are assigned to the second class (except Barnabas); the remainder of the saints have third class feasts or commemorations.
The last class of the saints relegated the lesser known saints to the status of a commemoration. The Mass may be that of the saint, or he may be commemorated in the Mass of the previous Sunday. Likewise, a lesser known saint may be commemorated in the Mass of a better known saint, something that often happens when a newly canonized displaces an older saint on his feast day.
Feasts associated with the Holy See are often higher in rank than they might otherwise be (e.g. Peter and Paul). Regional, diocesan, parish, and religious order patron saints are usually celebrated with a higher rank where appropriate.