According to our friends at Wikipedia: The modern holiday of Halloween may have 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 erroneously 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, 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, where the 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.

Did We Mention The Pumpkin?

And Our Friends At Wikipedia Come To The Rescue Again: The carved pumpkin, lit by a candle inside, is one of Halloween's most prominent symbols. These lanterns are usually carved from a turnip or swede (or more uncommonly a mangelwurzel). The jack-o'-lantern can be traced back to the Irish legend of Stingy Jack, a greedy, gambling, hard-drinking old farmer. He tricked the devil into climbing a tree and trapped him by carving a cross into the tree trunk. In revenge, the devil placed a curse on Jack, condemning him to forever wander the earth at night. This story has been passed down through generations of Irish families. The carving of pumpkins is associated with Halloween in North America, where pumpkins were readily available and much larger, making them easier to carve than turnips. Many families that celebrate Halloween carve a pumpkin into a frightening or comical face and place it on their home's doorstep after dark. In America the tradition of carving pumpkins is known to have preceded the Great Famine period of Irish immigration. The tradition of carving vegetable lanterns may have been brought over by the Scottish or English--documentation is unavailable to establish when or by whom. The carved pumpkin was originally associated with harvest time in general in America and did not become specifically associated with Halloween until the mid-to-late 19th century.