On All Souls' Day, Roman Catholics commemorate and pray for the holy souls in Purgatory, undergoing purification of their sins before entering heaven.
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.
O Almighty God, the God of the spirits of all flesh, who by a voice from heaven proclaimed, Blessed are the dead who die in the Lord: Multiply, we pray, to those who rest in Jesus the abundant blessings of your love, that the good work which you have begun in them may be made perfect unto the day of Jesus Christ. And of your mercy, O heavenly Father, grant that we, who now serve you on earth, may at last, together with them, be partakers of the inheritance of the saints in light; for the sake of your Son Jesus Christ our Lord.
* above painting - "All Souls' Day" by William Bouguereau