The celebration of Mass, the sacrifice of Calvary continued on our altars, has ever been for the Church the principal means of fulfilling towards the dead the great commandment of charity. Masses for the dead are found in the fifth century. But it was St. Odilo, fourth abbot of Cluny, who was responsible for the institution of the general commemoration of all the faithful departed; he instituted it and fixed its celebration on November 2, the day after All Saints. The practice spread to the rest of Christendom.
Daily in a special Memento in the Canon of the Mass, at which the priest remembers all those who have fallen asleep in the Lord, the priest implores God to grant them a place of happiness, light and peace. Thus there is no Mass in which the Church does not pray for the faithful departed; but today her thoughts are directed towards them in a particular fashion, with the maternal preoccupation of leaving no soul in purgatory without spiritual aid and of grouping them all together in her intercession. By a privilege that Benedict XV's decree has extended to the whole world every priest can today celebrate three Masses; for the liberation of the souls in purgatory the Church multiplies the offering of the sacrifice of Christ, from which she draws forever on behalf of all her children, infinite fruits of redemption.