Whoops, wrong picture. Behold!
Rannphairti anaithnid at English Wikipedia
We don't know why, but this is creepier than a real human skull.
Originally, jack-o-lanterns were carved out of turnips, whose waxy skin and gaunt form offer much more of an undead panache than the relatively jolly pumpkin. The origin of this, as befits the holiday, comes from a scary story. In 17th-century Ireland, there once lived a fellow named Stingy Jack, who, true to his name, was a bit of a son of a bitch. One day, Jack met the Devil in a pub (this is an Irish story) and decided to trick him. Playing up on his frugal nature, Jack convinced Satan to transform himself into a coin in order to pay for their drinks. However, instead of plopping the demonic coin on the counter, Jack decided to pocket it right next to a silver cross. Weakened by the divine symbol, the Devil could not escape and had to make a deal with Stingy Jack, promising to leave his soul alone for 10 years. We can only assume the Devil had been drinking since early that afternoon and was no match for Jack's deviousness.