All Saints’ Day, also known as All Hallows’ Day, or the Solemnity of All Saints, is one of the most important feast days of the Church and honours all the Saints known and unknown. It is celebrated on November 1st.
In Ireland it is a Holy Day of Obligation.
In Rome, All Saints Day was originally celebrated on May 13 and had its origins in the pagan observation of the Feast of the Lemures, in which the malevolent and restless spirits of the dead were propitiated.
In the 8th century Pope Gregory III changed the date to November 1st coinciding with the foundation 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”.
Nov 1st was chosen to coincide with the Celtic holiday of Samhain, with obvious similarities to Lemuria, but which was also a harvest festival.
However in Ireland, since we already had Samhain, we didn’t make the switch to November 1st, and historical documents [ the Felire of Oengus and the Martyrology of Tallaght ] prove that the medieval churches in Ireland continued to celebrate the feast of All Saints in the Spring (20 April.)
Protestants disagree with the idea of Saints as understood by the Church, and generally commemorate all Christians on All Saints’ Day; if they observe All Saints Day at all.