Many of our holidays have origins in Pagan traditions. Halloween is no different. This particular event is the modern version of the three-day Celtic festival, known as Samhain. This is a Gaelic word, pronounced ‘sah-win’ or ‘sow-win’, which means ‘summer’s end’. This was the Celt’s New Year, marking the end of both summer and the harvest while heralding the long winter ahead.
As winter was often associated with death, the the Celts believed the boundaries between worlds were blurred and the dead. As stated on Wikipedia: “Like Beltane/Calan Mai, it was seen as a liminal time, when the boundary between this world and the Otherworld thinned.” Druids built huge bonfires where sacrifices were made and people dressed in costumes in hopes of being avoided by the spirits they believed were present. “The period was also thought to be favourable for divination on matters such as marriage, health, and death.” (Britannica.com)
There are some interesting articles I thought you’d enjoy. They give a comprehensive history of Halloween from its origin to today’s version.