Here's some interesting info about the day:
The origins of Halloween are Celtic in tradition and have to do with observing the end of summer sacrifices to gods in Druidic tradition. In what is now Britain and France, it was the beginning of the Celtic year, and they believed Samhain, the lord of death, sent evil spirits abroad to attack humans, who could escape only by assuming disguises and looking like evil spirits themselves. The waning of the sun and the approach of dark winter made the evil spirits rejoice and play nasty tricks. Believe it or not, most of our Halloween practices can be traced back to these old pagan rites and superstitions.
Source:
Should Christians Celebrate Halloween?