A few little known facts–not sure if the bottom 2 are accurate or not — but they are interesting. (Source)
Halloween celebrates the Christian holiday of All Hallows Eve (followed by All Saints Day on November 1). But the Christian holiday is likely rooted in the Celtic holiday, Samhain, or a number of other pre-Christian harvest festivals.
In Great Britain, Jack-O-Lanterns are traditionally made from turnips. The Halloween custom came to America through Irish immigrants, and since turnips weren’t cheap state-side, Americans used pumpkins. Today, pumpkins are used worldwide, to the disappointment of turnip farmers everywhere.
The Jack-O-Lantern tradition comes from another Celtic tale. Jack tricked the Devil into paying for his drink, so the Devil gave Jack a hellish ember. But crafty Jack placed the ember safely into a turnip, which he carved and carried with him so as to scare away any future hellish encounters.