One early spelling of “Halloween” was “All Hallows’ Even (Even = evening). The “all” and “s” were dropped, “hallows’ ” and “even” became a closed compound, and the apostrophe took the place of the “v,” giving us “Hallowe’en”—just one of many transitional spellings along the way to “Halloween,” which the Oxford English Dictionary shows as first appearing in 1786.
Other spellings before “Halloween” included “Hallow-e’en,” “Alhollon Eue,” and “Halhalon evyn.”
It originated with the ancient Celtic festival of Samhain, when people would light bonfires and wear costumes to ward off ghosts. In the eighth century, Pope Gregory III designated November 1 as a time to honor all saints; soon, All Saints Day incorporated some of the traditions of Samhain. 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. Celts believed that on the night before the new year, the boundary between the worlds of the living and the dead became blurred. On the night of October 31 they celebrated Samhain, when it was believed that the ghosts of the dead returned to earth.
Some Info from History.com