On November 1st, the Church celebrates the Solemnity of All Saints, a Holy Day of Obligation, honoring all the Saints in heaven. Throughout the month of November, the Catholic Church also remembers the faithful departed. Offering Mass and praying for the dead has always been practiced since the early Church. 2Maccabees 12:42-45 (c164 B.C.) gives the earliest account that prayers (v. 42) and sacrifices (v. 43) for the dead are efficacious. There is also ample evidence of the custom of praying for the dead in the inscriptions of the catacombs in Rome (2nd to 5th centuries), with their constant prayers for the peace and refreshment of the souls of the departed and in the early liturgies, which commonly contained commemorations of the dead.
In the Communion of Saints, a link of charity exists between the faithful who have already reached their heavenly home (Saints in Heaven), those who are expiating their sins in purgatory (Souls in Purgatory) and those who are still pilgrims on earth (the faithful living on earth). Between them there is an abundant exchange of all good things. In this wonderful exchange, the holiness of one profits others, well beyond the harm that the sin of one could cause others. Thus, recourse to the Communion of Saints allows the contrite sinner to be more promptly and efficaciously purified of the punishments for sin. (CCC 1475) Since the faithful departed now being purified are also members of the same Communion of Saints, one way we can help them is to obtain indulgences for them, so that the temporal punishments due for their sins may be remitted. (CCC 1479)
The faithful living on earth pray for the souls in Purgatory, as the latter wait for the day when they will join the company of the Saints in Heaven. Offering a Mass is the highest means the Church can provide for charity for the dead, but also through prayers and penances. Another way the faithful living on earth can help them is to obtain indulgences for the souls in Purgatory, so that the temporal punishments due for their sins may be remitted.
A plenary indulgence, applicable only to the souls in purgatory, is granted to the faithful who, on any and each day from November 1 to 8, devoutly visit a cemetery and pray for the departed, and on All Souls’ Day, devoutly visit a church or an oratory and recite an Our Father and the Creed.
A partial indulgence, applicable only to the souls in purgatory, is granted to the faithful who, devoutly visit a cemetery and at least mentally pray for the dead and devoutly recite lauds or vespers from the Office of the Dead or the prayer Requiem Aeternam (Eternal rest).
In order to obtain the plenary indulgence, the faithful must be in the state of grace, have the interior disposition of complete detachment from sin (including venial sin), receive the sacrament of confession, receive the Holy Eucharist, and recite prayers for the intentions of the Holy Father (at least 1 Our Father and 1 Hail Mary). If some of the above conditions are not fulfilled, the indulgence becomes partial.