Halloween was Christianity`s failed attempt at eradicating paganism in the 9th Century.
Originally, the pagans dedicated the period of late October to early November as a time for honoring their ancestors. Then along came the Christians, who declared November 1st to be All Saints Day and forced the pagans to honor that day instead. The pagans simply celebrated their ancient traditions in secret on the evening before November 1st.
When the Medieval Era ended, and the age of Protestantism began, the pagan tradition was celebrated more openly, eventually becoming an evening holiday in the Western Hemisphere.