Halloween's origins can be traced back to the Celtic festival of Samhain. It marks the end of the Celtic year, with the new year occuring on November 1st. Many Christians are unaware of the historical significance, though it is easy to see that Halloween is a celebration of dark, evil, ghostly things.
Celtics believed that on Halloween, the worlds between the living and the dead could unite. They believed that the dead could return to earth and wreak havoc on mankind. On the same night, psychics and witches could increase their power and predict the future harvest. In order for the Celts to ensure their security and a positive prophecy, they dressed in costumes and sacrificed animals to the "living dead" at various festivals and parties.
When Europeans came to America, Halloween customs varied among the colonies. Protestant belief kept the traditions from becoming wide-spread. Yet, when the Irish were fleeing a famine and came to the colonies, they re-gnited Halloween's customs and so we have continued the traditions in the fifty states.
Consider the source!