Apparently, the Romans celebrated Pomona, the goddess of fruits, gardens and orchards as winter approached. Then, when the Romans took over the known world, the Celts converted their festival of Samhain to align with Rome's Pomona. Finally, not to be outdone, the Christians decided to hold 'All Hallows Eve' at the same time. And, if you say, 'All Hallows Eve' fast enough, it becomes 'Halloween'. So there you have it; it's Roman, not, like we all think, American.
While the idea didn't originate in America the way we celebrate it did. The Americans have adapted the old traditions to end up with the modern dress-ups, parties and trick-or-treating.