On Hallow's Eve, everyone alights the carved pumpkins at night, a practice that has been done years ago. There is, of course, a story that must be told, dating back to the term, "will-o'-the-wisp'.
The story goes with the Irish myth about a man called "Stingy Jack". Once upon a time, Stingy Jack invited the Devil to have a drink with him; and true to his name, Stingy Jack did not want to pay for his own drink, thus convinced the Devil to turn himself into a coin, so Jack could use it to buy their drinks. And so the Devil did turn into a coin, and Jack decided to keep the coin in his pocket next to a silver cross so that the Devil won't be able to change back in his original form.
On one condition that he would not bother Jack for a year, Jack decided to free the Devil. Should Jack die within the year, the Devil cannot claim his soul. The next year, Jack tricked the Devil into climbing a tree to pick a fruit. Jack carved a cross sign on the bark of the tree so that the Devil won't be able to come down. For the exchange that the Devil would be able to come down, he wouldn't botherJack for another 10 years. Of course, Jack died.
As far as the myth went, God did not want him in heaven. The Devil, upset by Jack's tricks on him, wouldn't allow him in Hell either. Instead, the Devil sent him to the dark night with only a burning coal to light his way. Jack put a coal in a carved turnip and roamed the entire Earth ever since. The Irish, then, believed that this figure is "Jack of the Lantern", or more now known as "Jack O'Lantern".
There have been many variations of this story since then, but take heed of caution, that whenever you come across a pumpkin head with a sinister smile, it's either you feed him with candy, or simply run for your life!