The origin of the festival of All Saints celebrated in the West dates to May 13, 609 or 610, when Pope Boniface IV consecrated the Pantheon at Rome to the Blessed Virgin and all the martyrs; the feast of the dedicatio Sanctae Mariae ad Martyres has been celebrated at Rome ever since. There is evidence that from the fifth through the seventh centuries there existed in certain places and at sporadic intervals a feast date 13 May to celebrate the holy martyrs. The origin of All Saints’ Day cannot be traced with certainty, and it has been observed on various days in different places. However, there are some who maintain the belief that it has origins in the pagan observation of 13 May, the Feast of the Lemures, in which the malevolent and restless spirits of the dead were propitiated. Liturgiologists base the idea that this Lemuria festival was the origin of that of All Saints on their identical dates and on the similar theme of “all the dead”.
The feast of All Saints, on its current date, is traced to the foundation by Pope Gregory III (731.741) of an oratory in St. Peter’s for the relics “of the holy apostles and of all saints, martyrs and confessors, of all the just made perfect who are at rest throughout the world”, with the day moved to 1 November and the 13 May feast suppressed.
This usually fell within a few weeks of the Celtic holiday of Samhain, which had a theme similar to the Roman festival of Lemuria, but which was also a harvest festival. The Irish, having celebrated Samhain in the past, did not celebrate All Hallows Day on this November 1 date, as extant historical documents attest that the celebration in Ireland took place in the spring: “…the Felire of Oengus and the Martyrology of Tallaght prove that the early medieval churches [in Ireland] celebrated the feast of All Saints on April 20.”
A November festival of all the saints was already widely celebrated on November 1 in the days of Charlemagne. It was made a day of obligation throughout the Frankish empire in 835, by a decree of Louis the Pious, issued “at the instance of Pope Gregory IV and with the assent of all the bishops”, which confirmed its celebration on November 1. The octave was added by Pope Sixtus IV (1471.1484