Chhath is one of the ancient vedic festivals , which is meant as an ode to the Sun God, Surya.
Celebrated largely by people of origin from Bihar, Jharkhand and Uttar Pradesh, Chaath is now a kind of national religious rite, celebrated in every major city. The ritual, which culminates in a homage to Agni and his younger wife, Usha, at waterbodies in the form of a puja done on succeeding days at sunset and sunrise. The austerities involved in Chaath Puja are intense and can be done by men or women, though women ( called Chaath vratins) usually lead the show.
No idols are usually worshipped on Chaath, though this year I note a trend of worshipping Surya murthis. Looks like a new fangled phenomenon. Chaath, dedicated to nature and using the gifts available in nature, is the most eco-friendly Indian festival.
In mythology, Bhagavan Rama performed Chaath Puja in the Ramayana age to propetiate Agni and Usha. Chaath finds mention in Mahabharata also, and Karna, one of the central characters of Mahabharata, was in fact the son of Surya.
Here is Chaath 2018 in pictures, including the Surya Mandir at Bundu near Jamshedpur.h