What we celebrate as Halloween has its roots in the Great Festival of Samhain (pronounced Sow-in) of pre-Christian Celtic times. The Celts were a culture thought to have migrated into Europe from somewhere north of the Black Sea about seven thousand years ago.(1) Its traditions are the most undiluted to this day in the country of Ireland.
Samhain was, and is, one of the four Great Festivals in the Celtic calendar. It's the end of summer, time for final harvest and to bring animals in from the fields. The old year is over, and all who've passed are honored -- along with the ancestors. The New Year is born with the light of a great bonfire. It's a crossroads time, an in-between, a void, when there's easy passage out of what the Celts called the Otherworld.
Today's Halloween is a remnant of the Celtic Fire festival, with add-ons from the later Roman and Christian church overlay. Around 7th Century AD, the Christian hierarchy began its demonization of the nature spirits and the "craft of the wise." A new holy day was created called Hallowmas or Allhallows, now called All Saints' or All Souls' Day. Over the centuries All Hallows' Eve became All Hallow e'en, which today we know as Halloween.
(1)Heroes of the Dawn, Celtic Myth, Time-Life Books: 1996.