Halloween was originally a Pagan Holy day called "All Hallows Eve." It was one of four, and the most powerful, holy days throughout the year when spirits could walk the earth and communicate with the living.
The word Halloween comes from All-Hallows-Eve as it is at the end off All-Hallows-day which is also know as All-Saints-Day. Although now All-Saints-Day falls one day after Halloween, they use to be celebrated together on the same day.
Halloween was originally called All Hallows' Eve which means the evening before All Saints' Day. "Hallow" is an Old English word for "saint". This was shortened to Halloween.