All Souls' Day has a datable beginning. About the year 998, Odilo, abbot of Cluny, decreed that on 2 November all the monasteries under Cluny should keep a solemn commemoration of the dead, Masses, psalms and almsgiving offered for every departed soul, placed deliberately on the day after All Saints so that the whole company of the dead, the glorified and the still-waiting, would be remembered together. From the great Cluniac network the custom spread through the dioceses of France and then across the Western Church, until by the later Middle Ages it was a general observance. Earlier prayer for the dead was ancient and widespread; what Odilo did was fix it to a single universal date and knit it to the feast of All Saints.
Liturgical historians note that Odilo's 998 decree fixed and universalized a practice rather than inventing it outright, since scattered local commemorations of the dead, including a custom recorded in Visigothic Spain by Isidore of Seville, predate the Cluniac decree. The popular claim that the November date descends from the pre-Christian Celtic festival of Samhain is treated with skepticism by specialists; the historian Ronald Hutton, in his study of the British ritual year, found no continuous documented link between Samhain and the medieval Christian commemoration, reading the connection as a much later folk association rather than an inherited survival.