The observance of All Saints’ Day dates back to the early centuries of the Christian church. Initially, Christians celebrated martyrs on the anniversary of their death. Because that day marked the birthday that would lead to eternal life with Jesus in heaven. However, as the number of martyrs increased during the persecutions of Emperor Diocletian and his successors, which lasted from 303 to 311 AD, it became necessary to assign a specific day to each witness who died for the faith. has become difficult. Therefore, in the early fourth century, the Church established a common day to honor all the martyrs in two cities in present-day Turkey: May 13 in Edessa and the first Sunday after Pentecost in Antioch.
In 609 or 610, Pope Boniface IV consecrated the Pantheon in Rome to the Virgin Mary and all the martyrs and fixed May 13 as the day to commemorate the saint. Later, Pope Gregory III (731-741) consecrated a chapel in St. Peter’s Basilica. They built a cathedral to all the saints and moved the celebration to November 1st. Finally, Pope Gregory IV (827-844) made November 1 the official day of the Feast of All Saints throughout the Western Church, thereby recognizing and honoring the same feast of Eastern Catholics at the beginning of the year.
-via Catholic Answers
