7 Therefore, since it is certain by the doctrine of the Catholic Church, that the souls detained in purgatory are benefited by the prayers of the faithful, and especially by the august Sacrifice of the Altar , We think we can give them no more useful and desirable pledge of Our love than by everywhere increasing the offering of the pure oblation of the Most Holy Sacrifice of Our Divine Mediator, for the extinction of their pain. We therefore decree, with all the necessary dispensations and indulgences, the last Sunday of next September as a day of ample expiation on which will be celebrated by Ourselves, and equally by each of Our brethren the Patriarchs, Archbishops, and Bishops, and also by other prelates exercising jurisdiction in a diocese, each in his own church, whether patriarchal, metropolitan, or cathedral, a special Mass for the Dead with the greatest solemnity possible, and according to the rite ordered in the Missal for the Commemoration of all Souls .
Source: Quod Anniversarius (Vatican.va)