Not coming from a Halloween background, I was intrigued by the history of pumpkin carving and the definition of a "Jack-O-Lantern". Interestingly, the whole concept is derived from an old Irish folk tale. There are many versions, but the most popular relates the tale of how "Stingy Jack", a most unsavory character, tricked the Devil by sending him up a tree to pick a piece of fruit. While he was up there, Jack carved a cross into the tree's bark which prevented the Devil from coming down until he promised to leave Jack alone and not claim his soul when he died. Soon after, Jack died and went to the Gates of Heaven. He was refused entry because of his treachery whilst alive. So he went to visit his old Friend in Hell and again was refused entry because the Devil kept his word and would not claim Jack's soul.
This left Jack in a dark and terrible place, so the Devil gave him a glowing ember to light his way. Jack placed this in a hollowed-out turnip and has been roaming the Earth with it ever since. The Irish began to refer to this ghostly figure as "Jack of the Lantern" and then simply "Jack O'Lantern".
On all Hallow eves, the Irish made their own version of Jack's lantern using hollowed out turnips, gourds or potatoes to ward off Stingy Jack and other evil spirits. It wasn't until the Irish came to America and realized that pumpkins were more readily available and easier to carve, that this became the common form of Jack-O-Lantern.