(WHTM) — Halloween occurs every year on October 31. But why does it occur on that day every year?

Halloween follows many of the traditions from the ancient Gaelic festival of Samhain. This festival would occur on Nov. 1, but it started the evening before on Oct. 31. It was also a time of year when the season changed.

But besides the seasons changing, it was also believed as the one time of the year when human life and the afterlife could be intertiwned. People in that time believed that they could connect with the dead, which is why, according to Country Living, Halloween is known as a ‘haunted’ holiday.

Country Living also said that Halloween also has Christian roots as well. In the eighth century, Pope Gregory II changed All Saints Day which was originally on May 13 to Nov. 1. Then in the next century, Pope Gregory IV added All Saints Day to the universal Christian calendar, which made the celebration worldwide.

With Halloween on Oct. 31, All Saints’ Day on Nov. 1, and All Soul’s Day on Nov. 2., this is sometimes collectively called Allhallowtide

Halloween is also sometimes called ‘All Saints’ Eve.