What vegetable were jack-o-lanterns originally made of?

Answer 1

Turnips were originally used. They were used because they looked like skeletons head. These were considered to contain (the heads) wisdom and the spirit and were stuck to the door to scare away the demons by the Celts. The Irish did it to remember Stingy Jack who wandered the earth with a turnip with a light in it because he either

1. wasn't accepted into heaven because of his stingy miserly ways and the devil helped him out

or

2. he tricked the devil into climbing a tree and carving a cross into the trunk. The devil cursed him to walk the earth eternally.

Answer 2

The vegetable was a turnip. It was a turnip because the Celts believed that the head contained wisdom and the energy of the body. They would "headhunt" them and stick them to the doors to frighten off harmful spirits.
Pumpkins are usually carved to make jack-o-lanterns.

Answer 3

The tradition comes from Ireland and the original lanterns were made from Turnips or Potatoes (pumpkins began to be used once the tradition moved to america).