All Souls' Day, also called The Commemoration of All the Faithful Departed, is a day of prayer and remembrance for the faithful departed, observed by Christians on 2 November. In Western Christianity, including Roman Catholicism and certain parts of Lutheranism and Anglicanism, All Souls' Day is the third day of Allhallowtide, after All Saints' Day (1 November) and All Hallows' Eve (31 October). Before the standardization of Western Christian observance on 2 November by St. Odilo of Cluny in the 10th century, many Roman Catholic congregations celebrated All Souls' Day on various dates during the Easter season as it is still observed in the Eastern Orthodox Church, the Eastern Catholic churches and the Eastern Lutheran churches. Churches of the East Syriac Rite (Assyrian Church of the East, Ancient Church of the East, Syro-Malabar Catholic Church, Chaldean Catholic Church), (Syriac Catholic Church). commemorate all the faithful departed on the Friday before Lent. As with other days of the Allhallowtide season, popular practices for All Souls Day include attending Mass offered for the souls of the faithful departed, as well as Christian families visiting graveyards in order to pray and decorate their family graves with garlands, flowers, candles and incense. Given that many Christian cemeteries are interdenominational in nature, All Souls Day observances often have an ecumenical dimension, with believers from various Christian denominations praying together and cooperating to adorn graves.

Known in Latin as Commemoratio Omnium Fidelium Defunctorum, All Souls' Day is known

In the Catholic Church, "the faithful" refers essentially to baptized Catholics; "all souls" commemorates the church penitent of souls in purgatory, whereas "all saints" commemorates the church triumphant of saints in heaven. In the liturgical books of the Latin Church it is called the Commemoration of All the Faithful Departed (Latin: Commemoratio omnium fidelium defunctorum). The Catholic Church teaches that the purification of the souls in purgatory can be assisted by the actions of the faithful on earth. Its teaching is based also on the practice of prayer for the dead mentioned as far back as 2 Maccabees 12:42–46. 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, alms, deeds, and especially by the sacrifice of the Holy Mass.

In the Lutheranism, "the whole people of God in Christ Jesus" are seen as saints and All Souls Day commemorates those believers who have died as the 'faithful departed'.

The United Protestant tradition emphasizes "the Christian belief in bodily resurrection and eternal life" in observances of All Souls Day.

All Souls Day is seen by many Christian leaders are one in which ecumenism is celebrated, given that believers from various denominations collectively visit Christian cemeteries that are interdenominational in nature. Christians from the Catholic, Lutheran, Reformed, Anglican, Methodist and Baptist denominations often come together to clean, repair and then decorate graveyards together. Ecumenical prayer services are often held at Christian cemeteries on All Souls Day.

In Western Christianity, there is ample evidence of the custom of praying for the dead in the inscriptions of the catacombs, with their constant prayers for the peace of the souls of the departed and in the early liturgies, which commonly contain commemorations of the dead. Tertullian, Cyprian and other early Western Fathers witness to the regular practice of praying for the dead among the early Christians.

In the sixth century, it was customary in Benedictine monasteries to hold a commemoration of the deceased members at Whitsuntide. In the time of St. Isidore of Seville (d. 636) who lived in what is today Spain, the Monday after Pentecost was designated to remember the deceased. At the beginning of the ninth century, Abbot Eigil of Fulda set 17 December as commemoration of all deceased in part of what is today Germany.