On the Traditional Practice of the Gregorian Mass

An ancient but mostly unknown custom of the Church is the offering of a daily Mass for 30 days for a soul in Purgatory. The custom of praying 30 days for the dead can be traced back to the Old Testament. The Jews prayed for 30 days after the death of Moses according to their "days of weeping" for the mourning rites (Deut. 34:8).

Gregorian Masses are named after St. Gregory the Great, Pope from 590 to 604 A.D., who is credited with promoting and popularising the practice. 

Before he was Pope, St. Gregory was a monk and abbot. A fellow monk, Justus, fell sick and when he saw that he was nearing death Justus disclosed to his brother, Copiosus, who had come to the monastery to care for him, that he had sinned by keeping hidden three gold coins. For the sake of Justus and the community, St. Gregory ordered that the ailing monk was to have no more contact with the brethren of the community, but only be tended by Copiosus, and that he was to be buried apart from the deceased brethren ("in a dunghill"), with his three coins. The severity of the sanctions caused Justus to die a contrite man, and put the fear of God into the monks.

A month later, however, St. Gregory pondered the torments Justus would be experiencing in Purgatory and ordered that one Mass be offered for Justus each day for 30 days. Time passed until Copiosus arrived at the monastery with a report that Justus had appeared to him with the news that he passed from Purgatory to Heaven on that day. The monks in turn informed Copiosus of the series of Masses, the last of which was offered on that day. St. Gregory concluded that the series of Masses delivered Justus from Purgatory. 

As a result "Gregorian Masses"; spread throughout Europe and many religious orders. It unfortunately faded after the Protestant Reformation and the French Revolution.

The Masses are to be offered once per day for 30 consecutive days for the intention of one person.  The stipend amount for such Masses is usually far more than the usual amount given, due to the burden it places on the one offering the Masses, as the Masses should be said without interruption. The exception to this is the Triduum during Holy Week, when the Masses may be stopped until Easter Sunday and then resumed on that day.