[The vigil of this feast is popularly called "Hallowe'en" or "Halloween". Etymology:
short for All Hallow Even (All Saints' Eve)
Solemnity celebrated on the first of November. It was instituted to honor all the saints, known and unknown, and, according to Urban IV, to supply any deficiencies in the faithful's celebration of saints' feasts during the year. In the early days the Christians were accustomed to solemnize the anniversary of a martyr's death for Christ at the place of martyrdom. In the fourth century, neighboring dioceses began to interchange feasts, to transfer relics, to divide them, and to join in a common feast. In the persecution of Diocletian the number of martyrs became so great that a separate day could not be assigned to each. But the Church, feeling that every martyr should be venerated, appointed a common day for all. At first only martyrs and St. John the Baptist were honored by a special day. Other saints were added gradually, and increased in number when a regular process of canonization was established.
The commemoration of all the faithful departed is celebrated by the Church on November 2, or, if this be a Sunday or a solemnity, on November 3. The theological basis for the feast is the doctrine that the souls which, on departing from the body, are not perfectly cleansed from venial sins, or have not fully atoned for past transgressions, are debarred from the Beatific Vision, and that the faithful on earth can help them by prayers, almsgiving and especially by the sacrifice of the Mass.
In the early days of Christianity the names of the departed brethren were entered in tablets. Later, in the sixth century, it was customary in Benedictine monasteries to hold a commemoration of the deceased members on Pentecost Sunday. Eventually the commemoration was moved to November 2, the day following All Saints Day.