It's been quite interesting reading about the origins of Halloween. It is the day before All Saints Day - which used to be called All Hallows. In Britain and Ireland in particular, pagan Celts celebrated the Day of the Dead on All Hallows Day (1st November). The spirits supposedly rose from the dead and, in order to attract them, food was left on the doors. To scare off the evil spirits, the Celts wore masks. Then when the Romans invaded Britain, they embellished the tradition with their own, which is the celebration of the harvest and honoring the dead. These traditions were then passed on to the United States and Canada.
If you go back even further with the Ancient Celts, the new year began around the 1st of November, a day referred to in modern Gaelic as Samhain ("Sow-in", meaning: November). Just as sundown meant the start of a new day, shorter days signified the start of the new year; so the harvest festival began every year on the night of October 31.
As November 1 is the first day of the new year, the day also meant the beginning of winter, which the Celts often associated human death with. The Celts also believed that on October 31 (the night before the new year), the boundary separating the dead from the living became blurred.
Of course, America has taken the tradition of Halloween to heart, going totally over the top as they do with everything!