The tradition reportedly began in Ireland, where they would actually carve faces into turnips, to represent spirits or goblins.
According to Wikipedia, folklorist Jabez Allies described his recollection of the tradition as follows:
“In my juvenile days I remember to have seen peasant boys make, what they called a ” Hoberdy’s Lantern,” by hollowing out a turnip, and cutting eyes, nose, and mouth therein, in the true moon-like style ; and having lighted it up by inserting the stump of a candle, they used to place it upon a hedge to frighten unwary travellers in the night.”
Want to learn more about Halloween? Check out Holiday Facts and Fun: Halloween, on Discovery Education Streaming!