It started as a pagan ritual; today it features kids dressed as Care Bears. Halloween began in Britain more than 2,000 years ago as a night when Celtic tribespeople communed with their dead. In the middle ages, the Roman Catholic Church "Christianized" it as Hallowmas - All Saints Day and All Souls Day. Halloween had a difficult trip to the New World: New England Puritans shunned it, though other colonies kept it alive. In the late 1700s, townwide "play parties" in the fall featured dancing and games. Waves of Irish immigrants in the mid-1800s brought Halloween traditions with them. These blended with African, German, English, and Scottish traditions. By the early 1900s, Halloween was a full-blown American holiday.
Source: 'Halloween: An American Holiday, and American History,' by Lesley Pratt Bannatyne (Pelican, 2000).