All Saints is celebrated on November 1.
Christians were accustomed to solemnize the anniversary of a martyr’s death for Christ at the place of martyrdom. In the fourth century, neighbouring dioceses began to interchange feasts, to transfer relics, to divide them, and to join in a common feast; as is shown by the invitation of St. Basil of Caesarea (397) to the bishops of the province of Pontus. Frequently groups of martyrs suffered on the same day, which naturally led to a joint commemoration. 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. The first trace of this we find in Antioch on the Sunday after Pentecost. We also find mention of a common day in a sermon of St. Ephrem the Syrian (373), and in the 74th homily of St. John Chrysostom (407). 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; still, as early as 411 there is in the Chaldean Calendar a “Commemoratio Confessorum” for the Friday after Easter. In the West Boniface IV, 13 May, 609, or 610, consecrated the Pantheon in Rome to the Blessed Virgin and all the martyrs, ordering an anniversary.
In Catholicism, All Saints’ Day is a Holy Day of Obligation in many (but not all) countries, meaning going to Mass on the date is required unless one has a good reason to be excused, such as illness. However, in a number of countries that do list All Saints’ Day as a Holy Day of Obligation, including England and Wales, the solemnity of All Saints’ Day is transferred to the adjacent Sunday, if 1 November falls on a Monday or a Saturday, while in the same circumstances in the United States the Solemnity is still celebrated on 1 November but the obligation to attend Mass is excused.