The custom of keeping holy one day in the year s the festival of all God”s saints, whether commemorated in the Liturgy or not, dates back to at least the beginning of the Fifth Century.
In ancient times it was usually called the feast of All Holy Martyrs. The day of it”s celebration varied; and in the East, even now, All Saints is in most places a movable feast. Pope St. Boniface IV, when dedicating the Roman Pantheon as the Church of Our Blessed Lady and All Holy Martyrs, appointed November 1 for the chief annual festival in the sacred building.
Gregory III built an oratory at St. Peter”s in honor of all the saints, confessors as well as martyrs, who had died in all parts of the world. Gregory IV chose November 1 as the Feast of All Saints. The Octave was added by Sixtus IV.
(Source: The New Roman Missal, Rev. F. X. Lasance)