All Saints’ Day, also called All Hallows’ Day, Hallowmas, or Feast of All Saints, in the Christian church, a day commemorating all the saints of the church, both known and unknown, who have attained heaven. It is celebrated on 1 November in the Western churches and on the first Sunday after Pentecost in the Eastern churches. In Roman Catholicism, the feast is usually a holy day of obligation.
The origin of All Saints’ Day cannot be traced with certainty, and it has been observed on various days in different places. A feast of all martyrs was kept on 13 May in the Eastern church according to Ephraem Syrus (died c 373), which may have determined the choice of 13 May by Pope Boniface IV when he dedicated the Pantheon in Rome as a church in honour of the Blessed Virgin and all martyrs in 609. The first evidence for the 1 November date of celebration and of the broadening of the festival to include all saints as well as all martyrs occurred during the reign of Pope Gregory III (731–741), who dedicated a chapel in St. Peter’s, Rome, on November 1 in honour of all saints. I
By 800, there is evidence that churches in Ireland, Northumbria (England) and Bavaria (Germany) were holding a feast commemorating all saints on 1 November. Some manuscripts of the Irish Martyrology of Tallaght and Martyrology of Óengus, which date to this time, have a commemoration of all saints of the world on 1 November.