How did Halloween become a holiday? Here's what to know about it

The line between the worlds of the living and dead will be their thinnest tonight. At least, that's what the old lore of Halloween says.

While now the day is mostly associated with trick-or-treating, costumes, candy and ghost stories, the origins of the holiday go back to the ancient Celtic festival of Samhain , according to the History Channel.

What is Samhain ?

About 2,000 years ago, the holiday of Samhain emerged.

At that point, Nov. 1 marked the New Year for the Celts, and with that came the belief that "on the night before the new year, the boundary between the worlds of the living and the dead became blurred," the History Channel said.

With the line so thin, the belief was ghosts were free to roam the Earth.

To protect themselves, "people would light bonfires and wear cost