The feast of All Saints goes back to the dedication of the Roman Pantheon, which Pope Boniface IV consecrated to the Virgin Mary and all the martyrs (Sancta Maria ad Martyres) on May 13, 610. After that, different churches began celebrating a feast in honor of all the saints, with more or less solemnity and on different dates. For example, the Irish monks observed such a feast on April 20 in honor of all the saints of Europe. Later, Alcuin promoted the celebration on November 1, and it seems that King Louis the Pious spread this date throughout his kingdom at the request of Pope Gregory IV (827–844).
In the East, Christians were ahead of the West in celebrating all the saints together. As early as the fourth century, the Syriac Church kept this feast during the Easter season, and the Byzantine Church celebrated it immediately after Pentecost.
