It is on All Souls' that we recall, and pray for, all of those Souls enduring the torments of Purgatory. Requiem aeternam dona eis, Domine, et lux perpetua luceat eis. Eternal Rest give unto them, O Lord, and let perpetual light shine upon them.
Dante next to the pit of Hell to the left, Florence to the right, and the Mountain of Purgatory behind.
The Old Catholic Encyclopedia has a good, if brief, article on the Feast of All Souls: Old Catholic Encyclopedia: All Souls' Day
The Fisheaters site, for its part, has some splendid details, especially of traditional customs: Fisheaters: All Souls' Day
Of course, the indulgence for the Poor Souls' continues today, through 8 November, for visiting a cemetery; likewise, there is a particular indulgence today, All Souls' Day, for visiting a Church or Oratory and praying for the dead, along with an Our Father and Creed. Note the Enchiridion Indulgentiarum:
"29 Pro fidelibus defunctis
§ 1. Plenaria indulgentia, animabus in Purgatorio detentis tantummodo applicabilis, conceditur christifideli qui
1° singulis diebus, a primo usque ad octavum novembris, coemeterium devote visitaverit et, vel mente tantum, pro defunctis exoraverit;
2° die Commemorationis omnium fidelium defunctorum (vel, de consensu Ordinarii, die Dominico antecedenti aut subsequenti aut die sollemnitatis Omnium Sanctorum) ecclesiam aut oratorium pie visitaverit ibique recitaverit Pater et Credo."
The details, in English, are as follows: Plenary Indulgence for Cemetery Visit. Also, on this day, for Visiting a Church or Oratory.
Father Zuhlsdorf, on his blog, has a great discussion of these indulgences: Fr. Z's Blog: All Souls' Day Indulgences
Image of Purgatory by Carracci.
Looking, then, at Purgatory, as its existence, and recalling the souls detained therein, is the basis of this feast!
The Catechism of the Catholic Church summarizes the teaching on Purgatory as follows:
III. The Final Purification, or Purgatory
1030 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.
1031 The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned.604 The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent. the tradition of the Church, by reference to certain texts of Scripture, speaks of a cleansing fire:605
As for certain lesser faults, we must believe that, before the Final Judgment, there is a purifying fire. He who is truth says that whoever utters blasphemy against the Holy Spirit will be pardoned neither in this age nor in the age to come. From this sentence we understand that certain offenses can be forgiven in this age, but certain others in the age to come.606
1032 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."607 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.608 The Church also commends almsgiving, indulgences, and works of penance undertaken on behalf of the dead:
Let us help and commemorate them. If Job's sons were purified by their father's sacrifice, why would we doubt that our offerings for the dead bring them some consolation? Let us not hesitate to help those who have died and to offer our prayers for them.609
604 Cf. Council of Florence (1439): DS 1304; Council of Trent (1563): DS 1820; (1547): 1580; see also Benedict XII, Benedictus Deus (1336): DS 1000.605 Cf. ⇒ 1 Cor 3:15; ⇒ 1 Pet 1:7.606 St. Gregory the Great, Dial. 4, 39: PL 77, 396; cf. ⇒ Mt 12:31.607 ⇒ 2 Macc 12:46.608 Cf. Council of Lyons II (1274): DS 856.609 St. John Chrysostom, Hom. in 1 Cor. 41, 5: PG 61, 361; cf. ⇒ Job 1:5.
[Original text of the Catechism can be found here: Vatican: Catechism of the Catholic Church]
Likewise, the Old Catholic Encyclopedia has a rather informative article on the subject: Old Catholic Encyclopedia: Purgatory
Finally, in the Appendix of the Supplement to the Summa Theologiae of St. Thomas Aquinas, there is a discussion of purgatory, including: Question 2 on the Souls in Purgatory; Appendix II: on Purgatory. The articles that follow add a few more details.
Of course, one of the greatest literary works of the West, in this bloggers opinion, the Divine Comedy of Dante. How many other authors have a Papal Encyclical about their work? (cf. Benedict XV, In Praeclara Summorum). While not a theological manual, the Divine Comedy does embody a Catholic worldview in his presentation. So, why not go check out the final section of the work that fits with our post? Here is a copy of the Purgatorio, which is my own favorite in the Divine Comedy trilogy: Dante's Purgatorio
Finally, perhaps one of the most brilliant of all Gregorian Chants, is the Sequence for All Souls Day, the Dies Irae:
Live well, so as to avoid Purgatory, or at least minimize our time there!