Chhat Puja vrat katha or Legend or the origin of Chhat Puja (Dala Chhat) is associated with Mahabharat katha. Mahabharat has the reference of Chhat pooja. It is also mentioned that Chhath or Dala Chhat was started in the time of Mahabharat.
Chhat Puja and Draupadi:
Draupadi, wife of the Pandavas, has a great power to cure any type of disease including leprosy and other severe disorders. The main factor behind this was she worships Sun God (Surya Bhagwan) with utmost devotion.
Sun rays have power of curing any disease. The celestial and divine energy of Surya made her most powerful woman in the world to cure any severe disease. As a commemoration of Draupadi and Surya Puja, the main Deity to worship on Chhat Puja is Surya.
Chhat Pooja and Karna:
Chhat Pooja or Surya Sashti vrat is observed by King Karna, the son of Surya, who was renowned warrior and well-known for his nature of charity.
Since Karna became a popular King by observing Surya Puja, Hindus also worship Surya dev for courage, fame, health, wealth and prosperity.