The original idea of halloween or, to use its original name, All Hallow's Eve, was that because November 1st was All Saints Day, the day when saints were remembered and honored for their goodness, all the evil spirits thought they better get their oar in and whoop it up before the saints got busy with inspiring and helping humanity. Hence the spooky costumes, trouble making and general misrule the night before.
Over they years it has moved from a celebration by the general population, to a holiday for kids' mischief (Trick or Treat really had the threat of an often rather destructive trick), to a tamed down, candy orgie for kids and an excuse to dress up for everybody; adults getting back into the picture.
Nowadays even celebrities, like Heidi Klum, and media giants like TODAY are seizing it as an opportunity to show off and get publicity. TODAY article >