The traditional worship of Surya, the sun God, is the prime part of Chhath Puja. According to some of the scholars, the tradition of this ancient festival is older than the history of the hymns of Vedas. Rig Veda described similar styles and rituals of worshipping Sun. People who have read the Hindu epic Mahabharata, they know that Draupadi, one of the prime characters of the epic, used to follow same kind of rituals.
It is known that Draupadi and the Pandavas, the rulers of Hastinapur (modern Delhi), celebrated and followed the rites of Chhath Puja on the advice of the sage, Dhaumya. It was due to the Draupadi’s prayers and worship of the Sun God, the Pandavas could salvage their lost kingdom. Darupadi was also blessed and rewarded. All her immediate quandaries were solved.
Some opine that Karna, the son of Surya, had started the tradition of worshipping the Sun God. He had ruled over Anga Desh (Bhagalpur district of Bihar) during the period of Mahabharata. The intrepid fighter Karna fought hard against the strong Pandavas in Kurukshetra war.
The Vedic history of it dates back to the ancient Vedic times. Renowned saints during that time not used to take food. They used to energize themselves with the sun rays. All these were done devotionally by Chhath method.
It is also believed that the exciting Chhath Puja started at the time when local emperors used to ask and invite the expert mega purohits (modern days known as Shakya Dwipi Brahmins) to worship Surya. The tradition of the Chhath Puja was actually started by them. In recent times, the cultures and mores of the Chhath Puja are followed by the devotees of those places where these Shakya Dwipi Brahmins are found.