Once upon a time, a man named Stingy Jack invited the Devil to drink with him. When Jack didn't want to pay for the drinks, he persuaded the Devil to turn into a coin to pay. When the Devil turned into a coin, Jack put the money into his pocket with a silver cross. The silver cross prevented the Devil from changing back. When Jack decided to finally let him go, he made him promise not to bother him for a year after and if he died, he wouldn't take his soul. So, the Devil promised and a year went by. One day, Jack convinced the Devil to climb a tree for a piece of its fruit, but when he did, Jack carved a cross into the tree's bark. The Devil was fooled once again, and could not get out of the tree. Jack made him promise to leave him alone for another ten years. The Devil succumbed and was let down. After many years, Jack died. God would not allow him into Heaven after making so many deals with the Devil, and the Devil, bitter after so many tricks, wouldn't let Jack into Hell. He sent Jack away with only a single burning coal for light into a dark night. Jack put his coal into a hollowed turnip and walked the land alone this way forever. He became known as "Jack of the Lantern" in Ireland.
To frighten away Stingy Jack and other evil spirits, people began to make Jack's lanterns in Ireland and Scotland out of turnips and potatoes with scary faces carved into them. They placed them in windows or by doors. Once immigrants came to America, they found that pumpkins make even better "jack-o-lanterns", and they've been used for decorations ever since.
For other interesting information and some fantastic facts about pumpkins, click here.