Because you need to know things like this...
Halloween has its origins in the ancient Celtic festival known as Samhain. The festival of Samhain is a celebration of the end of the harvest season in Gaelic culture, and is sometimes regarded as the "Celtic New Year". Traditionally, the festival was a time used by the ancient pagans to take stock of supplies and slaughter livestock for winter stores. The ancient Gaels believed that on October 31, now known as Halloween, the boundary between the alive and the deceased dissolved, and the dead become dangerous for the living by causing problems such as sickness or damaged crops. The festivals would frequently involve bonfires, into which bones of slaughtered livestock were thrown. Costumes and masks were also worn at the festivals in an attempt to mimic the evil spirits or placate them.
History of name
The term Halloween is shortened from All Hallows' Even (both "even" and "eve" are abbreviations of "evening", but "Halloween" gets its "n" from "even") as it is the eve of "All Hallows' Day", which is now also known as All Saints' Day. It was a day of religious festivities in various northern European Pagan traditions, until Popes Gregory III and Gregory IV moved the old Christian feast of All Saints' Day from May 13 (which had itself been the date of a pagan holiday, the Feast of the Lemures) to November 1. In the ninth century, the Church measured the day as starting at sunset, in accordance with the Florentine calendar. Although All Saints' Day is now considered to occur one day after Halloween, the two holidays were, at that time, celebrated on the same day. Liturgically, the Church traditionally celebrated that day as the Vigil of All Saints, and, until 1970, a day of fasting as well. Like other vigils, it was celebrated on the previous day if it fell on a Sunday, although secular celebrations of the holiday remained on the 31st. The Vigil was suppressed in 1955, but was later restored in the post-Vatican II calendar.
(from the Wikipedia entry on Halloween...)