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The Origins of Halloween: "The earliest trace (of Halloween) is the Celtic festival, Samhain, which was the Celtic New Year. It was the day of the dead, and they believed the souls of the deceased would be available".

Samhain (pronounced sah-win or sow-in) means "summer's end" by the Celts. In old Germanic and Celtic societies, what we call equinoxes and solstices marked the middles of the season, not the beginnings." Therefore if there exist an autumnal equinox, winter solstice, spring equinox and a summer solstice, there are also the beginning of autumn, winter, spring and summer. All of these eight dates were important. Summer's end which meant the beginning of winter was an important time for people who survived on plants grown in the field and animals that were kept in pastures."This day marked the end of summer and the harvest and the beginning of the dark, cold winter, a time of year that was often associated with human death." It is most likely this reason that the Druids (Celtic pagans) believed that the spirits of those who died the preceding year roamed the earth the night of Samhain.

Source: http://www.albany.edu/~dp1252/isp523/halloween.html