An ancient Vedic solar ritual unique to Bihar and eastern India, dedicated to Surya (the sun god) and his consort Usha (dawn). Devotees stand waist-deep in water at sunrise and sunset, offering arghya (water) to the sun. Chhath is one of the oldest surviving Vedic rituals, predating Puranic Hinduism, with roots in the Rigvedic worship of Surya as the visible form of Brahman. The four-day austerity includes 36-hour waterless fasting.