why and what started the tradition of carving pumpkins on halloween?

Answer 1

The Irish brought the tradition of the Jack O'Lantern to America. The original Jack O'Lantern was not a pumpkin. The legend goes back hundreds of years in Irish History.
Jack O'lanterns weren’t always Pumpkin , and Halloween wasn’t always Halloween. This celebration began as a pagan festival called Samhain, when Celts would walk around with various root vegetable lanterns to guide the roaming dead to the next world. They also carved spooky faces into them (the lanterns, not the dead) to keep malignant spirits away.