About.com's Catholicism site:
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 now are in Purgatory, being cleansed of their venial sins and atoning before entering fully into Heaven. The importance of the feast was made clear by Pope Benedict XV (1914-22), 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.All Souls' Day is not a holy day of obligation, but many use this solemn feast day to remember their friends and family members who have gone from this life.
Prayer for Eternal Rest:
Eternal rest grant unto them, O Lord, and let perpetual light shine upon them. May the souls of the faithful departed, through the mercy of God, rest in peace. Amen.