All Saints' Day is a Christian holy day observed by many Western churches on November 1 and by Eastern churches on the first Sunday after Pentecost. The day now honors all saints of the church, even those not known by name.
The first All Saints' Day occurred on May 13, 609 (C.E.) when Pope Boniface IV accepted the Pantheon as a gift from the Emperor Phocas. Boniface dedicated it as the Church of Santa Maria Rotonda in honor of the Blessed Virgin and all martyrs. During Pope Gregory III's reign (731-741), the festival was expanded to include all saints and a chapel in St. Peter's church was dedicated accordingly. Pope Gregory IV officially designated the day in 837.
1. World Book Encyclopedia, 1992.