Carved pumpkins were originally carved turnips and rutabagas meant to scare off evil spirits and also referenced a folk tale about a greedy man named Jack.

Carving pumpkins became a thing when British and Irish people began colonizing North America and began using pumpkins instead, which had the advantage of being bigger, easier to carve, and more abundant in the Americas than turnips or rutabagas.

While it's often popular to try to connect these practices to ancient Celtic polytheism, the truth is the actual evidence is quite lacking. Not much is actually known about any ancient Celtic religion or its practices.