All Saints’ Day | Definition, History, Catholic, Holy Day of Obligation, Observances, & Facts – Britannica

Our editors will review what you’ve submitted and determine whether to revise the article.
All Saints’ Day, in the Christian church, a day commemorating all the saints of the church, both known and unknown, who have attained heaven. It is celebrated on November 1 in the Western churches and on the first Sunday after Pentecost in the Eastern churches. In Roman Catholicism, the feast is usually a holy day of obligation. The day is primarily observed by Christians with a tradition of venerating the saints, such as Roman Catholics, Orthodox Christians, and Anglicans, but certain other churches use it as a day to honor the faithful departed more generally. It is part of the three-day triduum—sometimes known as Allhallowtide—dedicated to remembering the dead, beginning with Halloween (October 31) and followed by All Saints’ Day (November 1) and All Souls’ Day (November 2).
The origin of All Saints’ Day cannot be traced with certainty, and it has been observed on various days in different places. A feast of all martyrs was kept on May 13 in the Eastern church according to Ephraem Syrus (died c. 373), which may have determined the choice of May 13 by Pope Boniface IV when he dedicated the Pantheon in Rome as a church in honor of the Blessed Virgin and all martyrs in 609. The first evidence for the November 1 date of celebration and of the broadening of the festival to include all saints as well as all martyrs occurred during the reign of Pope Gregory III (731–741), who dedicated a chapel in St. Peter’s, Rome, on November 1 in honor of all saints. In 800 All Saints’ Day was kept by Alcuin on November 1, and it also appeared in a 9th-century English calendar on that day. In 837 Pope Gregory IV ordered its general observance. In medieval England the festival was known as All Hallows, and its eve is still known as Halloween.
In 1914 Pope Benedict XV issued his first encyclical, Ad beatissimi Apostolorum, on the All Saints’ Day to appeal for peace among the nations engaged in World War I.
In Roman Catholic and certain Anglo-Catholic churches, masses held on All Saints’ Day incorporate the Litany of the Saints, a call-and-response prayer that invokes Mary, the Apostles, and other saints. Many churches have a dedicated “book of remembrance” in which the names of departed loved ones are written throughout the year, with the names being read aloud during All Saints’ services. Candle lighting and other remembrances are common, and the Resurrection is a frequent theme for homilies. Some churches and families hold festive All Saints’ parties in which children dress up as famous saints. In the United States, these gatherings are sometimes seen as a religious alternative to Halloween.
In Mexico and other parts of Latin America, All Saints’ Day is incorporated into Day of the Dead celebrations. Churches may encourage families to bring photos of their loved ones, and prayers are offered to the holy dead. Special requiem masses are sometimes held and processional walks are common. Some people undertake a nightly novenario (novena)—a spiritual devotion of reciting of a set form of prayer for nine consecutive days—concluding on All Saint’s Day.
In Eastern Orthodoxy, All Saints’ Day typically includes veneration of the All Saints icon, which depicts Christ surrounded by a circle of saints, including the Theotokos, Michael the archangel, St. John the Baptist, the Apostles, and important martyrs and ascetics. The day honors the known and unknown saints throughout history.
Both All Saints’ and All Souls are observed in many places with visits to the cemetery. Loved ones may use the holiday as a time to clean tombs and grave sites, often adding decorations and candles. Memories of the departed, as well as prayers, are shared during these visits. 

source