Significance
Chhath Puja is a four-day festival of thanksgiving to the Sun God (Surya) and Chhathi Maiya, seeking well-being, prosperity and progeny. It is the great folk festival of Bihar, Jharkhand, eastern UP and Nepal.
Renowned for its rigour and purity, it is one of the most demanding vows in Hinduism.
Rituals
The four days are Nahay Khay, Kharna, Sandhya Arghya (offering to the setting Sun) and Usha Arghya (offering to the rising Sun). Devotees observe a nirjala fast, stand in water offering arghya with thekua and fruit, and worship on riverbanks and ghats.
How the date is fixed
The main Sandhya Arghya falls on the Shashthi (sixth tithi) of Kartika Shukla paksha, with the Usha Arghya at sunrise the next morning; the arghya times track your local sunset and sunrise.