Halloween has its roots in the ancient Celtic festival of Samhain, a pagan religious celebration that marked the end of the harvest season and the beginning of winter. During Samhain, the veil between the worlds of the living and the dead was thought to be especially thin, so people could communicate with loved ones who had passed on. The festival also included lighting bonfires to ward off ill-intentioned spirits.

When the Christian church began to spread throughout Europe, it attempted to replace pagan holidays like Samhain with its own religious observances. All Saints Day on November 1 served as a time for the living to pray for the souls of the dead, while All Hallows Eve on October 31 became Halloween.

Halloween began in the United States sometime around the 19th century when the Irish and Scottish immigrants brought their celebrations, including All Saints’ Day, with them. Halloween really took off in the 1900s when retailers began mass producing cards, decorations, and Halloween costumes. Trick-or-treating began as a bribe, and it picked up speed in the United States in the 30s and 40s, leading to todays Halloween ritual of ringing doorbells and shouting "Trick or treat" in hopes of scoring candy and other treats.

In summary, Halloween has its roots in the ancient Celtic festival of Samhain, and it evolved over time with the influence of Christian beliefs and practices. Halloween became a major holiday in America after mass Irish and Scottish immigration in the 19th century, and it really took off in the 1900s when retailers began mass producing Halloween-related items.