As we all know, the practice pf praying and sacrifice for the dead goes back to the Jewish custom of sacrifice for the dead. This custom of prayers and sacrifices for the dead predate the birth of our Lord Christ in Bethlehem. We find an explicit Old Testament description and exhortation of prayer and sacrifice for the dead in 2 Maccabees 12:42-26. Moreover, all the Church Fathers testify to the practice of praying for the dead in the Eucharistic liturgy. Praying for the dead an example of one of the most ancient Christian customs - older than Christmas, older than church buildings, even older than the New Testament itself.
But what about this special day, All Souls, on which we pray and sacrifice for the dead in a special way? We find an account of its origins in the writings of Saint Peter Damian, who records the following story.
In the 900s, there was a French pilgrim returning from the Holy Land who was shipwrecked on an island with a cave from which belched heat and gas (maybe a volcano of some sort). This pilgrim met a Christian hermit who lived near this cave. The hermit explained that he could sometimes overhear demons in the cave complaining about all the souls that are released from purgatory through the prayers and sacrifices of the monks in Cluny, France.
When the French pilgrim returned to France, he visited the monastery of Cluny and recounted the hermit's story to the abbot of the monastery, then Abbot Odilo. The pilgrim testified to the great number of souls delivered from purgatory through the humble prayers of the Cluniac monks. Odilo (died in 1048) was deeply moved by this and redoubled the monks efforts in assisting the souls in purgatory. Thus, he dedicated the day after All Saints Day (Nov 1) to all the souls still in purgatory (Nov 2). Soon, the practice spread to the rest of France and then to the universal Church so that November 2nd became All Souls Day.
Saint Odilo, pray for us.
May the souls of the faithful departed through the mercy of God rest in peace.