From priest reader:
I just started praying the usus antiquior low mass after a year of preparation (of latin and the rubrics).
Question: When may I opt to say a missae defunctorum? When may I not? In other words, may a missae defunctorm be chosen over a 3rd class feast, etc.?
First and foremost, thank you for learning the older form, traditional form of Mass. I am always happy to learn that another priest is not ignorant of our common Latin Rite. Kudos to you. I am also hoping that you are young, so that you can say this form of Mass for many many years for many many people.
There are those special occasions as in, for example, when people die or it is the anniversary of their death. However, I think what you are aiming at is really the daily Requiem Mass.
The rules for these daily Requiem Masses are along the lines of the rubrics for votive Masses.
A Requiem is of the 2nd class on the day of death (or getting news of the death) or on the day of burial. That’s the Missa in die obitus seu depositionis defunctorum. It is of the 3rd class on the third, seventh, and thirtieth day after the death or burial.
A "daily" Requiem is 4th class and can be used on ferial days of the 4th class.
So… tomorrow 16 September is the Feast of Ss. Cornelius and Cyprian. It is a 3rd class feast. But the day after tomorrow, 17 September, is a "dies non", a 4th class feria. You can say the "daily" Mass for the Dead, the Missa quotidiana defunctorum.
I hope that helps.
And remember that there are some differences in the Requiem Mass. You exclude the Iudica me at the beginning, and do not bless the water. Kiss the altar at the end, but don’t give the final blessing before the Last Gospel. Leave off the gloria after washing your fingers and exclude the first of the three private prayers before your Communion. The Agnus Dei is a little different and your say Requiescat in pace for the dismissal.
So, Father, put on those nice black vestments. Put those unbleached candles on the altar and fire ‘em up!