Chhaṭh Pūjā — thanks to the Sun
Kārtika Śukla Ṣaṣṭhī (four days, Caturthī to Saptamī) · October–November
Chhaṭh is the great festival of Bihar, Jharkhand, eastern Uttar Pradesh and Nepal's Terai, held six days after Dīpāvalī. It thanks Sūrya, the Sun, and Chhaṭhī Maiyā (Ṣaṣṭhī Devī), protector of children, for life, health and harvest. It is famously strict and pure, with no idol and no priest required: the devotee — most often the woman of the house, though men keep it too — stands in water and offers arghya directly to the setting and the rising Sun.
It unfolds over four days. Nahāy-Khāy: after a bath in a river or at home, a single pure meal of rice, chana dal and lauki. Kharnā: a day-long fast broken at evening with kheer made with jaggery and roṭī, shared as prasāda; then a nirjala fast of about 36 hours begins. Sandhyā Arghya: at sunset the family carries bamboo baskets (dauri, sūp) of thekuā, fruit, sugarcane and coconut to the water, and the vratī offers milk and water to the setting Sun while women sing Chhaṭh songs. Uṣā Arghya: before dawn everyone returns, offers arghya to the rising Sun, and the fast is broken. Those who cannot reach a river can create a small pool on the roof or in the courtyard — the devotion is the same.
On the ghāṭs, safety matters: stand in shallow water only, keep children close and supervised, use designated ghāṭs with lifeguards, and do not leave plastic or offerings in the river. Chhaṭh is now celebrated wherever people from Bihar and Purvanchal live — Delhi's Yamunā, Mumbai's Juhu beach, Kolkata — and in Mauritius, Fiji, Trinidad and Suriname. A smaller Chhaṭh is also kept in Caitra.
Tradition offers several stories. One says that Karṇa, son of Sūrya, stood waist-deep in water every day to worship his father, and from him came the custom of arghya in water. Another says that Draupadī and the Pāṇḍavas kept the Chhaṭh vrata during their troubles and regained their kingdom. Folk tradition in Munger tells that Sītā, after returning to Ayodhyā with Rāma, kept the Sūrya-ṣaṣṭhī fast on the banks of the Gaṅgā. Behind all of them stands the very old Vedic reverence for the Sun as the visible giver of light and life.
Sūrya · Chhaṭhī Maiyā (Ṣaṣṭhī Devī) · 36-hour nirjala fast; arghya in water · Bihar, Jharkhand, eastern UP, Nepal Terai · Ṛgveda 1.50 (hymn to Sūrya)