Though the 30 consecutive Masses in the Gregorian series need not be celebrated by the same priest, nor at the same altar, they must each be offered for the same departed person for each of the consecutive 30 days. As you will appreciate, few priests by reason of their work are free and able to offer the 30 consecutive Masses of the Gregorian series without interruption. Hence, it requires extra time to arrange to have the Gregorian Masses scheduled as this cannot be done usually in a place where only one priest is stationed; in case he falls ill, there must be at least one other priest available, and free to continue the Masses without interruption.
Gregorian Mass Series
30 consecutive Masses for one deceased person