The All Saints’ Day (officially the Solemnity of All Saints and also called All Hallows or Hallowmas) is a solemnity celebrated on 1 November by parts of Western Christianity, and on the first Sunday after Pentecost in Eastern Christianity, in honor of all the saints, known and unknown.
In Western Christian theology, the day commemorates all those who have attained the beatific vision in Heaven. It is a national holiday in many historically Catholic countries. In the Roman Catholic Church, the next day, All Souls’ Day, specifically commemorates the departed faithful who have not yet been purified and reached heaven. Catholics celebrate All Saints’ Day and All Souls’ Day in the fundamental belief that there is a prayerful spiritual communion between those in the state of grace who have died and are either being purified in purgatory or are in heaven (the church penitent and the church triumphant, respectively), and the church militant who are the living. Other Christian traditions define, remember and respond to the saints in different ways.
The origin of the festival of All Saints celebrated in the West dates to May 13, AD 609 or 610, when Pope Boniface IV consecrated the Pantheon at Rome to the Blessed Virgin and all the martyrs; the feast of the dedicatio Sanctae Mariae ad Martyres has been celebrated at Rome ever since. This is of course an objection to the claim of many anti-Catholics that the holy observance was only copied from the pagan Roman Feast of the Lemures, in which the malevolent and restless spirits of the dead were propitiated. Hence, this May 13 feast has already been traditionally observed by the Christians as early as 4th century at Edessa according to St. Ephrem the Syrian and got no relation to paganism.
The feast of All Saints, on its current date, is traced to the foundation by Pope Gregory III (AD 731-741) of an oratory in St. Peter’s for the relics “of the holy apostles and of all saints, martyrs and confessors, of all the just made perfect who are at rest throughout the world”, with the day moved to 1 November and the 13 May feast suppressed. This usually fell within a few weeks of the pagan Celtic holiday of Samhain, which had a theme similar to the Roman festival of Lemuria, but which was also a harvest festival. However, contrary to the claim of the anti-Catholics that the Catholic observance was also based on the pagan festivity, the Irish, having celebrated Samhain in the past, did not actuallt celebrate All Hallows Day on this November 1 date, as extant historical documents attest that the celebration in Ireland originally took place in the spring: “…the Felire of Oengus and the Martyrology of Tallaght prove that the early medieval churches [in Ireland] celebrated the feast of All Saints on April 20.”
A November festival of all the saints was then widely celebrated on November 1 in the days of Charlemagne. It was made a day of obligation throughout the Frankish empire in AD 835, by a decree of Louis the Pious, issued “at the instance of Pope Gregory IV and with the assent of all the bishops”, which confirmed its celebration on November 1. The octave was added by Pope Sixtus IV (AD 1471-1484).
Also, in AD 998, St. Odilo, the abbot of the powerful Benedictine monastery of Cluny in southern France, ordered the commemoration of all the faithful departed, who still needed to be completely purified in Purgatory, to be held annually in the monasteries of his congregation on every 2nd of November (the day after the Solemnity of All Saints), which was traditionally observed on October 1 in Germany. Thence, the November 2 observance spread among the other congregations of the Benedictines and among the Carthusians, from France to the rest of Christendom. This feast, called All Souls’ Day.
On the other hand, the Eastern Christians of the Byzantine Tradition follow the earlier tradition of commemorating all saints collectively on the first Sunday after Pentecost, All Saints’ Sunday.
The feast of All Saints achieved great prominence in the ninth century, in the reign of the Byzantine Emperor, Leo VI the Wise (AD 886-911). His wife, Empress Theophano, commemorated on December 16, lived a devout life. After her death in 893, her husband built a church, intending to dedicate it to her. When he was forbidden to do so, he decided to dedicate it to “All Saints,” so that if his wife were in fact one of the righteous, she would also be honored whenever the feast was celebrated. According to tradition, it was Leo who expanded the feast from a commemoration of All Martyrs to a general commemoration of All Saints, whether martyrs or not.
NOTE: In the Church of Rome, 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 from that obligation, such as illness.