Halloween is celebrated on the eve of Samhein, November 1st, the Celtic pagan festival.
It was in 835 AD that the Roman Catholic Church changed the day of Samhein into a Christian festival day. It was to be known as the Feast of All Saints, All Saint's Day.
The old word meaning saint or holy person was "Hallow", used in "Hallowed be thy name" from the lord's prayer. Because of ancient Celt rituals that celebrated November the first as the New Year, New Years Eve was celebrated on October 31st. The modern associateion of witches and evil spirits comes from the time when evil spirits were thought to roam the night. The celebrations were held to frighten away these evil spirits and cleanse the community for the coming New Year. Christianity adopted the festival and made it into Halloween.
Unfortunately, for some, it is also a festival celebrating witchcraft and the black arts.