The origin of Halloween dates back to the Celtic festival of Samhain. On October 31st, the Celtics marked the end of their calendar year by building large, sacred bonfires, wearing costumes, and telling each other's fortunes. They believed that on this day, the boundary between life and death became blurred and the ghosts of the dead returned to earth to damage crops and cause trouble. In the 800's the Pope declared November 1st as All Saints' Day (also called All-hallows day) as a way to replace the Celtic festival and sanction the celebration as a religious holiday. The date October 31st became known as All-hallows Eve, and eventually, Halloween.
In modern times, Halloween is generally considered secular and is celebrated across the United States as well as many countries around the world including Ireland and Great Britian. People celebrate by wearing costumes, throwing parties, carving jack-o-lanterns, making haunted houses, and of course trick-or-treating. The tradition of trick-or-treating most likely dates back to the practice of placing of food outside the front door to ward off the ghosts of the dead.
On Halloween, people dress in a variety of costumes, including everything from witches, skeletons, devils, and black cats, to princesses, pirates, rock stars and cartoon characters. While Halloween is a time for horror and haunts, it is widely celebrated by children and is considered a time to celebrate.