Carving vegetables into scary faces then lighting them is an odd kind of custom that just must have interesting roots.
The Irish do not disappoint. Folks there have a legend of a drunken farmer, whose dealings with the devil led to him being turned back at both the gates of heaven and hell upon his death. Forced to wander the darkness of purgatory, Jack made an o'lantern from a turnip and lump of burning coal to guide his lost soul. Every Halloween, communities in Ireland would craft their own turnip lamps to scare him and other wayward spirits away.
The tradition became pumpkin-ified when Irish immigrants found the orange veggie sprouting in abundance in North America. Today, pumpkins mean big business at Halloween: U.S. farmers grow over a billion pounds a year, worth about $106 million.