The modern form of Halloween is thought to have developed from the Celtic celebration of Samhain (pronounced “Sah-wen”). The Celts are an ancient civilization that live in what is now Ireland, Wales, Scotland and parts of England, but once was spread out over almost all of Europe, as far a field as modern-day Turkey.
Samhain was a New Year celebration; the Celts counted a day from sundown to sundown and celebrated the coming New Year on the eve of November 1. They would light bonfires as part of the celebration to help guide the spirits of all the people who died in the last 12 months to the otherworld. The Celts believed the spirits would be given access to the otherworld during Samhain and would leave food and drinks out for these wandering spirits. One way they acquired food and drinks was to go from house to house, dressed up, and perform in exchange for treats, and in imitation of the wandering spirits they would sometimes play pranks or tricks.
As the Celts converted to Christianity, many of the traditional Celtic folk holidays were combined with Christian celebrations. In the sixth century, November 1 became the Feast of All Saints, or All Hallows, and the day before was called Hallows Eve, which eventually became Halloween.
I found the information for this using KDL’s access to the Gale Virtual Reference Library, one of the many awesome online databases KDL offers. Just navigate to the KDL website, click “Reference Resources,” then “Research Databases” and chose a subject category to see some of the databases KDL cardholders can access.