Halloween is known as an evil day of ghosts, goblins, and serial killers. But, from this pagan festival came a holy day – and its story is among one of the most interesting developments in history.

The term Halloween is a contraction of the words that spelled out “All Hallows Eve,” the night before All Hallows Day (or All Saints Day in modern vernacular). In the year 835, Pope Gregory III moved the Catholic Church’s holy days of All Saints Day to Nov. 1 and All Souls Day to Nov. 2, respectively.