It started as an ancient pre-Christian Celtic festival of the dead.

See, for the Celts back then, November 1st was their New Year's—that was the day that they switched from fall to winter.

It's also the time of the year that they believed the veil between the world of the living and the world of the dead was at its thinnest.

That meant that the dead could hang out with the living—that's not comfortable for anybody.

They had a festival called "Samhain".

It was the most significant holiday of their entire year.

They believed it was on Samhain the souls of all the people that had died that year finally travel to the other world.

To help them get there, the living people lit bonfires to honor the dead to help them along their journey and also to keep them away from the living.

In the early centuries of the first millennium, Christian missionary showed up—they wanted to change the religious practices of the Celts.

But rather than just obliterate the existing pagan holidays such as Samhain, they tried to transform it.

So what the missionaries did was basically say, "Hey, all those supernatural DD's that you worship, they're evil, they're demonic, they're spirits, they're scary.

Then they slapped the new Christian holiday on November 1st—All Saints Day.

So now everyone was supposed to worship the Saints on November 1st instead of the dead during the festival of Samhain.

It didn't work—All Saints Day became known as All Hallows.

But it was the night before All Hallows—the eve of All Hallows—that people continued to celebrate in the spirit of Samhain as a time of the wandering dead.

Eventually, All Hallows Eve became Hallow Evening, which eventually became Halloween.

Basically, an ancient Celtic, pre-Christian New Year celebration that, over the last couple thousand years, lost its pagan context and transformed into one of the largest secular holidays of modern times.