A Gregorian Mass for the dead is a very ancient practice dating back to Pope St. Gregory the Great in the 6th century. The custom involves offering a mass each day on behalf of a deceased loved one for 30 consecutive days. While typically requested soon after someone dies, a “Gregorian Thirty” aka “Gregorian Mass” aka “Thirty Masses” may be offered on behalf of a soul in Purgatory at any time. You can find more information about this custom, its spiritual effects, and its legitimacy at these sites (here, here, and here).
The Shrine of St. Jude acknowledges the gift a series of 30 consecutive Masses and its spiritual graces with a 5.5″ x 8.5″ commemorative card.