Also known as the Feast of All Saints, it is a day that commemorates all the saints in the Christian church. During the reign of Pope Gregory III in 731-741, he dedicated a chapel in St. Peter’s in honour of all the saints. This is the first evidence of celebrating the saints’ day on November 1.
Prior to this, holding a feast for all the saints have already been observed. During the 4th century, there was a commemoration of all the martyrs. In the time of Pope Boniface IV, he rededicated a shrine– Pantheon– to all the martyrs. The purpose of the rededication is so “that the memory of all saints might in the future be honored in the place which had formerly been dedicated to the worship not of gods but of demons.” (On the Calculation of Time).
