Gregorian Masses, so named because the practice was first popularized by Pope St. Gregory the Great, are a series of thirty Masses said for the soul of a deceased person. St Gregory relates in his Dialogues how, when he had finished the series of 30 Masses for a departed monk, the monk appeared to him to tell him he had thus gained entry into the glory of heaven on completion of the Gregorian Masses. This hallowed tradition has been declared "a pious and reasonable belief of the faithful" on the authority of the Sacred Roman Congregation of Indulgences.
There are a number of simple requirements to gain this benefit for a departed loved one.
- It is required that thirty consecutive Masses be said; i.e., one per day for thirty days;
- They must be said on thirty consecutive days WITHOUT interruption;
- They can only be offered for one deceased person; not for several and not for all the faithful departed;
- It is not required that these Masses be said by the same priest, but, they must be said for thirty consecutive days, whether by one, two or more priests, all for the same intention.
Catholic Mission Office
640 N. LaSalle Street
Chicago, IL 60610-4356
You can also find more sources here, and here.
UPDATE : I now use Clear Creek Monastery for Gregorian Masses. The monks there are very gracious, and you will receive a letter from the superior as well as a Mass card outlining the dates for your Gregorian Masses. The Masses are said in the extra ordinary form, the TLM.