Prayers for the dead were an ancient tradition, even in some pagan cultures.
In Jewish Scriptures, Judas Maccabeus “made atonement for the dead” (2 Mc 12:46), and early Christians inscribed prayers for their dead in Rome’s catacombs.
Local Catholic churches started remembering all the faithful departed in the early Middle Ages, with a Spanish feast from the time of St. Isidore of Seville, who died in 636.
The fixed November date is generally attributed to St. Odilio of Cluny, who decreed in 998 that all monasteries under the Cluny rule should sing the Office of the Dead the day after the feast of All Saints.
This custom gradually spread throughout Europe and was adopted by Rome in the 13th century.
Text: Catholic News Service