World Book Encyclopedia (Quoted in the Atlanta Journal on 10/16/1977):
“It was the Celts who chose the date of October 31 as their new year’s Eve and who originally intended it as a celebration of everything wicked, evil and dead. Also during their celebration they would gather around a community bonfire and offer as sacrifice their animals, their crops, and sometime themselves. And wearing costumes made from the heads and skins of other animals, they would also tell one another’s fortunes for the coming year.
“The celebration remained much the same after the Romans conquered the Celts around 43 A.D. The Romans did, however, add a ceremony honoring their goddess of fruit and trees and thus the association with apples and the custom of bobbing for them.”
Source and read arguments why Christians should not celebrate Halloween: http://www.letgodbetrue.com/bible/holidays/halloween.php