Halloween is a Celtic Tradition much older than American. It is a celebration of the change of the seasons and celebration of the dead. According to legend, on "All Hallows Eve" the world of the dead and the world of the living are not securely separated and ghosts / spirits come into the land of the living. Spirits both good and evil.
Believers go to the graveyard to invite recently deceased spirits home for the evening. Other believers dress up in scary costumes to keep the "evil spirits" away from them. The idea that new death is "bitter with the sweet." death being bitter and candy as sweet to help console is olden also. The hope of most is that ghosts / spirits satisfied will not do harm to the home. "Trick or Treat" is no joke to some people. Harmful events have happened on this day in the past including.the burning of coffins,
Halloween is also a night when those who believe they are witches or warlocks have more spiritual power to do their spells. In olden days, religious groups shun the holiday celebrations stating they were "Satanic." Now days, most religious groups have October Feasts to celebrate the changing of the seasons. Festivals are fun filled with costume parties,scary stories, food and festival..