"This teaching is also based on the practice of prayer for the dead, already mentioned in Sacred Scripture: "Therefore [Judas Maccabeus] made atonement for the dead, that they might be delivered from their sin." From the beginning the Church has honored the memory of the dead and offered prayers in suffrage for them, above all the Eucharistic sacrifice, so that, thus purified, they may attain the beatific vision of God. The Church also commends almsgiving, indulgences, and works of penance undertaken on behalf of the dead..."
Catechism of the Catholic Church 1032
The last day to submit a picture is on Wednesday, October 23.
The annual celebration is the last day of Allhallowtide, after All Saints' Day (1 November) and Halloween (October 31).
All Souls Day has a long history. As early as the ninth century, it was a custom for monasteries to set aside a day to pray for their dead, and an abbot of the Benedictine monastery of Cluny was the first to establish November 2 as the day for commemoration of the departed during the 10th century. Prior to the standardization of Catholic observance on 2 November, many Catholic congregations celebrated All Souls Day on various dates during the Easter season, as it is still observed in some Eastern Orthodox Churches and associated Eastern Catholic and Eastern Lutheran churches.
From the Handbook of Indulgences:
Lord Jesus, lovingly welcome into Your presence my deceased family, relatives and friends who have completed their journey on this earth, and have gone to their rest in the hope of rejoicing with you forever in heaven.
Source of all life and love, in Your infinite mercy, look favorably upon their goodness, and forgive any sins they may have committed, so they may enjoy for all eternity the great rewards You have promised to those who believe in You. Amen.
V. Eternal rest grant unto them, O Lord.
R. And let the perpetual light shine upon them.
May the souls of all the faithful departed, through the mercy of God, rest in peace. Amen.
V. Requiem aeternam dona eis, Domine.
R. Et lux perpetua luceat eis.
Fidelium animae, per misericordiam Dei, requiescant in pace. Amen.
Indulgences are a sharing in the treasury of the merits of Christ and the Saints, which the Church applies to the remission of the temporal guilt of sin, whose eternal guilt has already been forgiven in the Sacrament of Reconciliation (by the merits of Christ’s Passion, Death and Resurrection).
Even forgiven sins can still have temporal guilt that requires reparation to justice. If not satisfied in life, it will need to be satisfied in Purgatory (Mt. 5:25-26). The Church, therefore, speaks of indulgences as an effect of the tribunal of mercy, the Sacrament of Penance (CCC 1471). An indulgence is a mercy given by Christ through the Church when we do certain acts of prayer, penance, or charity specified by the Church. Besides the intrinsic value of the act before God in remitting temporal guilt, the Church attaches an additional value through the power of the keys given to St. Peter (Mt. 16:13-18).
A Catholic may gain an indulgence for his or herself or for the Poor Souls, but not for another living person—whose will alone determines their receptivity to God’s mercy.
Source: EWTN