"Memento Mori" - "Remember Death"
Often overshadowed by Halloween (October 31) and All Saints Day (November 1), All Souls Day is a solemn feast in the Roman Catholic Church commemorating all of those who have died and are in Purgatory, being cleansed of their venial sins and atoning before entering into Heaven. The importance of the feast was made clear by Pope Benedict XV when he granted all priests the privilege of celebrating three Masses on All Souls Day: one for the faithful departed; one for the priest's intentions; and one for the intentions of the Holy Father. Only on a handful of other very important feast days are priests allowed to celebrate more than two Masses.
On All Souls Day, we not only remember the dead, but we apply our efforts, through prayer, almsgiving, and the Mass, to their release from Purgatory. There are two indulgences attached to All Souls Day, one for visiting a church and another for visiting a cemetery. While the actions are performed by the living, the merits of the indulgences are applicable only to the souls in Purgatory.
Always refer to the Norms on Indulgences for specific guidelines and requirements for gaining plenary and partial Indulgences.
Praying for the dead is a Christian obligation. The Church devotes the month of November to prayer for the Holy Souls in Purgatory, and participation in the Mass of All Souls Day is a good way to begin this month.
Rorate Caeli outlined the indulgence conditions much more eloquently:
§ 1. A plenary indulgence, applied exclusively to the souls in Purgatory, is granted to the Christian faithful who:
1° on every single day, from the first to the eighth day in November, devoutly visit a cemetery and, even if only mentally, pray for the faithful departed;
2° on the day of Commemoration of All Faithful Departed [November 2] (or, according to the Ordinary, on the preceding or subsequent Sunday, or on the day of the solemnity of All Saints) piously visit a church or oratory and there recite the Pater and the Credo.