Purgatory

Purgatory[1]

Father Gerard, that the custom of having thirty masses said for the dead is also widely spread in Italy and other Christian countries. These Masses are called the Thirty Masses of St. Gregory, because the pious custom seems to trace its origin back to this great Pope. It is thus related in his Dialogues (Book 4, chap. 40): A Religious, named Justus, had received and kept for himself three gold pieces. This was a grievous fault against his vow of poverty. He was discovered and excommunicated. This salutary penalty made him enter into himself, and some time afterwards he died in true sentiments of repentance. Nevertheless, St. Gregory, in order to inspire the brethren with a lively horror of the sin of avarice in a Religious, did not withdraw the sentence of excommunication: Justus was buried apart from the other monks, and the three pieces of money were thrown into the grave, whilst the Religious repeated all together the words of St. Peter to Simon the Magician, Pecunia tua tecum sit in perditionem—“Keep thy money to perish with thee.” Sometime afterwards, the holy Abbot, judging that the scandal was sufficiently repaired, and moved with compassion for the soul of Justus, called the Procurator and said to him sorrowfully, “Ever since the moment of his death, our brother has been tortured in the flames of Purgatory; we must through charity make an effort to deliver him. Go, then, and take care that from this time forward the Holy Sacrifice is offered for thirty days; let not one morning pass without the Victim of Salvation being offered up for his release.” The Procurator obeyed punctually. The thirty Masses were celebrated in the course of thirty days. When the thirtieth day arrived and the thirtieth Mass was ended, the deceased appeared to a brother named Copiosus, saying, “Bless God, my dear brother, today I am delivered and admitted into the society of the saints.” Since that time the pious custom of celebrating thirty Masses for the dead has been established.

A Guide to Plenary Indulgences for the All Souls' "Octave", November 1-8

Visiting a Cemetery: An indulgence, applicable only to the Souls in Purgatory, is granted to the faithful, who devoutly visit a cemetery and pray, even if only mentally, for the departed. The indulgence is plenary each day from November 1-8; on other days of the year it is a partial indulgence.

Visiting a Church on November 2: A plenary indulgence, applicable only to the souls in purgatory, is granted to the faithful who, on All Souls' Day (or, according to the judgment of the ordinary, on the Sunday preceding or following it, or on the solemnity of All Saints), devoutly visit a church or an oratory and recite an Our Father and the Creed.

Praying for the Faithful Departed: 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;
devoutly recite lauds or vespers from the Office of the Dead or the prayer Eternal rest. (Manual of Indulgences, fourth edition, 1999)

Indulgence Requirements:

To gain a plenary indulgence, in addition to excluding all attachment to sin, even venial sin, it is necessary to perform the indulgent work and fulfill the following three conditions: sacramental Confession, Eucharistic Communion, and prayer for the intention of the Sovereign Pontiff.
A single sacramental confession suffices for gaining several plenary indulgences, but Holy Communion must be received and prayer for the intention of the Holy Father must be recited for the gaining of each plenary indulgence.
The three conditions may be fulfilled several days before or after the performance of the prescribed work; it is, however, fitting that Communion be received and the prayer for the intention of the Holy Father be said on the same day the work is performed.
The condition of praying for the intention of the Holy Father is fully satisfied by reciting one Our Father and one Hail Mary. A plenary indulgence can be acquired only once in the course of the day, a partial indulgence can be acquired multiple times.
If a visit to a Church or an oratory is required to obtain an indulgence attached to a particular day, this may be accomplished from noon of the preceding day until midnight of the particular day.

Manual of Indulgences, fourth edition, 1999 (Enchridion Indulgentarium)

[1]Schouppe S.J., Rev. Fr. F. X.. Purgatory Explained (with Supplemental Reading: What Will Hell Be Like?)