Gregorian Masses derive their name from St. Gregory the Great, the first Benedictine Pope who ruled the Church from year 590 to 604. Gregorian Masses are offered for thirty (30) consecutive days for the repose of the soul of a particular person.
St. Gregory was the first to have a series of Masses said for a departed soul. The event that started this pious practice took place while he was abbot of St. Andrews' Monastery in Rome, prior to his election to the papacy.
In the Fourth book of his Dialogues, St. Gregory relates how one of the monks of his monastery, named Justus, did not keep his vow of poverty very well, and who, with permission of his superiors, was also a doctor. The following is taken from the “Holy Souls” by Parente.
“On one occasion he accepted a certain sum in gold, and that was a grave sin against poverty. For it he was excommunicated. Afterwards he was so humiliated by the reports and by his penalty that he became gravely ill and died. However, he had confessed and made his peace with God. St. Gregory had the monk buried in a rubbish dump and he buried the gold in the grave with these words of St. Peter. `Your money will perish with you’ (Acts 8, 20). Some time later he felt moved with pity for the poor monk. He said, `Our departed brother has been tormented for long time by the pains of Purgatory. Charity tells us to free him. So go and starting from today offer the sacrifice of the Mass for him for a period of thirty days. Do not fail to offer the propitiatory Host even once for his liberation.’ The monks obeyed, but because he was occupied did not bother to count the days. One night the deceased person appeared to him and told him that he had been freed from the sufferings of Purgatory and had gone to Paradise. It was then that the days were counted and it was discovered that the number of Masses was 30. This was the origin of the Gregorian Masses for the deceased.