Although Halloween has its roots in ancient Celtic superstition, Halloween as a holiday is deeply American in its traditions. Having evolved into a celebration with no religious basis, Halloween has become a holiday whose purpose is pure fun.
In the Celtic world, (Ireland, the U.K., and northern France 2,000 years ago), the year began with the onset of winter, and ended when families celebrated the harvest with feasting and bonfires, giving thanks for the crops that would see them through the coming year. Lanterns were carved out of large turnips and lighted to guide the revelers to the feast. Bonfires were lit and sacrifices were made in thanksgiving for plentiful crops, and Priests dressed up in ceremonial animal heads and skins. It was a holiday for courting couples as well. Hopeful suitors came bearing sweets for the younger children in hopes of a few minutes of privacy with their sweethearts.
Colonial America celebrated Halloween with dances, singing, fortune telling, ghost stories, and mischief making, but due to the rigid Protestant belief system in the New England Colonies, these activities were limited mostly to Maryland and the southern colonies. By the 1850s Autumnal and harvest celebrations were common, but Halloween was not universally celebrated in the U.S.
The influx of Irish immigrants fleeing the Potato Famine had begun to have a lasting effect on how Americans celebrated Halloween. These immigrants brought with them a holiday celebration with deep Celtic roots including dressing in costume, telling stories about the dead, carving pumpkins, and handing out treats.
In the late 18th Century, religious and civic leaders led a movement to turn Halloween towards community and family get-togethers and away from witches, pranks and ghosts. Newspaper editorials urged parents to minimize the frightening and grotesque, and consequently, by the end of the 19th century, Halloween had lost most of its superstitious and religious connotations. From then until World War I, Halloween was celebrated largely with house parties for children and adults. Friends and families would gather at each others' homes where the children would bob for apples and the adults would play bridge or mahjong. Parties featured games, festive costumes, and seasonal foods. By the 1950s Halloween had evolved into a celebration for children. To combat rampant Halloween vandalism, the practice of Halloween bribery or Trick-or-treating was revived. This was seen as an inexpensive way for the entire community to share in the celebration, and in theory, the handing out of treats prevented the playing of tricks. Today, America spends approximately $6.9 billion annually on Halloween.
Reference: History.com