The Commemoration of the Faithful Departed, or All Souls' Day, evolved onto the church calendar long after All Saints' Day. Sometime between 998 and 1030, St. Odilo, the abbot at the Benedictine monastery in Cluny, France, encouraged all the monks to pray for the souls of those who had died, those awaiting the joys of heaven. He instituted this commemoration on the day after All Saints' Day, and soon other religious orders and churches began, on that same date, to annually remember all who had died.
Remembering and praying for the faithful departed is tied directly to our belief in purgatory. On All Souls' Day the universal church prays for all those in purgatory, people who were much like us, whose offense may have been less than ours. By pleading for them, we are inspired to lead purer lives.