All Saints’ Day, All Hallows, or Hallowmas begins at Vespers on the evening of 31 October – which, of course, is known as the Eve of All Hallows, Halloweven or Hallowe’en, and ends at sundown on 1 November.
This is the feast commemorating the saints, all those who have attained the beatific vision in Heaven.
The following day, unless it is a Sunday, is called All Souls’ Day.
The Feast of All Souls is a day of prayer for all the Faithful departed who have not yet been purified in the fires of Purgatory and reached Heaven, remembering that there is a union of prayer between those in Heaven (the "Church triumphant"),those in Purgatory (the "Church suffering") and the living (the "Church militant").
The Church Suffering are taken from Purgatory into Heaven to join the Church Triumphant
Some scholars believe that the Feast of All Saints may have been a christianisation of the ancient Roman observation of 13 May, the Feast of the Lemures, in which malevolent and restless spirits of the dead were propitiated.
In the persecution of Diocletian the number of martyrs became so great that a separate day could not be assigned to each and the Church, feeling that every martyr should be venerated, appointed one day for all.
|Pope St Boniface IV|
Pope St Gregory III (731–741) founded an oratory in St Peter’s Basilica for the relics of all the apostles and saints and moved the feast day from 13 May to 1 November.
Pope St Gregory III