Why we carve Jack-o'-lanterns

There is a long tradition of carving lanterns from vegetables throughout
 Ireland and Britain, but immigrants to North America used the native pumpkin.
Turnips were carved into lanterns in Ireland as a way of remembering
the souls in purgatory. They were left on the doorsteps on All’s Hallows Eve along with a
treat of some sort to ward off evil spirits.

They became known as Jack-o’-lanterns based on the legend of an Irishman
 named Stingy Jack. Apparently, Jack tricked the devil (the manner in which he tricked him varies,
 sometimes Jack has the devil hanging from an apple tree in others he is a thief)
and made a bargain with him that he would never take his soul. 

As time moved on ol’ Jack died, as all living things do, but he was too sinful to go to Heaven.
The devil however upheld his end of the deal and didn’t claim Jack either.
So Jack was left to wander the earth.

Jack had no where to go and no light to guide him. The devil gave him an ember from hell
 that would burn eternally and Jack placed it in a hallowed out turnip
and endless wonders the earth looking for a place to rest.