The feast of All Saints on November 1 is the day Christians honor the memory of the multitude of saints without their own feast day, who now enjoy the vision of God for all eternity. On November 2, All Souls Day, the Church reminds us of the need to pray for those who have died and are now being purified, so that they may attain the beatific vision of God.
From earliest times, the Church has honored the memory of the deceased and offered suffrages for them, in particular the Eucharistic sacrifice. The Church also recommends alms, indulgences and works of penance for the deceased. Here are some texts on this custom to pray for the deceased.
Catechism of the Catholic Church
“When the Lord comes in glory, and all his angels with him, death will be no more and all things will be subject to him. But at the present time some of his disciples are pilgrims on earth. Others have died and are being purified, while still others are in glory, contemplating 'in full light, God himself triune and one, exactly as he is’” (Lumen gentium 49). All of us, however, in varying degrees and in different ways share in the same charity towards God and our neighbors, and we all sing the one hymn of glory to our God. All, indeed, who are of Christ and who have his Spirit form one Church and in Christ cleave together (CCC, 954).
“In full consciousness of this communion of the whole Mystical Body of Jesus Christ, the Church in its pilgrim members, from the very earliest days of the Christian religion, has honored with great respect the memory of the dead; and ‘because it is a holy and a wholesome thought to pray for the dead that they may be loosed from their sins’ she offers her suffrages for them” (Lumen gentium, 50). Our prayer for them is capable not only of helping them, but also of making their intercession for us effective (CCC, 958).
All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven (CCC, 1030).
Purgatory shows God's great mercy and washes away the defects of those who long to become one with Him (Furrow, 889).
The holy souls in purgatory. Out of charity, out of justice, and out of excusable selfishness—they have such power with God!—remember them often in your sacrifices and in your prayers. May you be able to say when you speak of them, “My good friends the souls in purgatory” (The Way, 571).
At the beginning I felt the company of the souls in purgatory very strongly. I felt them as if they were tugging me by the cassock, asking me to pray for them and to ask them for their intercession. Since then, due to the great services they have done for me, I like to preach about and foster in souls this reality: my good friends the souls in purgatory (from The Way: Critical-Historical Edition, edited by Pedro Rodriguez, p. 740).
On the Communion of the Saints: I believe in the Communion of Saints and the forgiveness of sins
On the Christian meaning of death and eternal life: I believe in the resurrection of the body and life everlasting