All Saints Day was first commemorated in the fourth century for the feast of "all the martyrs," St. Ursula Catholic Church in Parkville, Maryland wrote in its bulletin. 

"In the early seventh century, after successive waves of invaders plundered the catacombs, Pope Boniface IV gathered up some 28 wagon-loads of bones and reinterred them beneath the Pantheon, a Roman temple dedicated to all the gods," the church wrote in the bulletin. "The pope rededicated the shrine as a Christian church. According to Venerable Bede, the pope intended 'that the memory of all the 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)."

When the Pantheon was dedicated, it was in May, which is when many Eastern churches still honor all saints (during Easter of Pentecost).

"How the Western Church came to celebrate this feast, now recognized as a solemnity, in November is a puzzle to historians. The Anglo-Saxon theologian Alcuin observed the feast on Nov. 1 in 800, as did his friend Arno, Bishop of Salzburg. Rome finally adopted that date in the ninth century," the church said in the bulletin.