On All Souls' Day, Roman Catholics commemorate and pray for the holy souls in Purgatory, undergoing purification of their sins before entering heaven. It is celebrated on November 2.
The theological basis for this feast is the doctrine that the souls which, on departing from the body, are not perfectly cleansed from venial sins, or have not fully atoned for past transgressions, cannot not yet attain the Beatific Vision, and that the faithful on earth can help them by prayers, alms, deeds and especially by the sacrifice of the Mass.
In the early days of Christianity the names of the departed brethren were recorded on tablets. Later, in the sixth century, the Benedictine monasteries commemorated their deceased during Pentecost. In Spain, there was such a day on Saturday before before Pentecost, at the time of St. Isidore (d. 636). In Germany there existed a ceremony of praying to the dead on 1 October. St. Odilo of Cluny (d. 1048) ordered the commemoration of all the faithful departed to he held annually in the monasteries of his congregation. From there, it spread among the other congregations of the Benedictines and among the Carthusians to various dioceses.
On this day, three requiem Masses are celebrated: one for the celebrant, one for the departed, and one for the Holy Father.
All About All Souls' Day
Prayer for the Poor Souls in Purgatory
V. Eternal rest grant unto them, O Lord.
R. And let the perpetual light shine upon them.
And may the souls of all the faithful departed, through the mercy of God, rest in peace. Amen.
Prayer of St. Gertrude the Great
Eternal Father, I offer Thee the most precious blood of Thy Divine Son, Jesus, in union with the Masses said throughout the world today for all the Holy Souls in Purgatory.
All Souls’ Day Indulgence -- November 2 Plenary Indulgence
From the Handbook of Indulgences
Visiting a Church or an Oratory on All Souls Day
A plenary (“full”) indulgence, which is applicable only to the souls in Purgatory is granted to the Christian faithful who devoutly visit a church or an oratory on (November 2nd,) All Souls Day.
Requirements for Obtaining a Plenary Indulgence on All Souls Day (Nov. 2nd)
1. Visit a church and pray for souls in Purgatory
2. Say one “Our Father” and the “Creed” in the visit to the church
3. Say one “Our Father” and one “Hail Mary” for the Holy Father’s intentions
4. Worthily receive Holy Communion (ideally on the same day if you can get to Mass)
5. Make a sacramental confession within a week of All Souls Day
6. For a plenary indulgence be free from all attachment to sin, even venial sin (otherwise, the indulgence is partial, not plenary, “full”).