Chhath is an ancient Hindu Vedic festival celebrated in Madhesh region of Nepal as well as all over Nepal. The Chhath Puja is dedicated to the Sun and his wife Usha in order to thank them for bestowing the bounties of life on earth and to request the granting of certain wishes.
The word Chhath means sixth  in Nepali, Maithili and Bhojpuri languages and the festival is celebrated on the sixth day of the month Kārtika of the Hindu luni-solar Bikram Sambat calendar. The word is a Prakrit derivation from the Sanskrit ṣaṣṭhi, meaning sixth.
The very first day of Chhath starts exactly 4 days from Tihar and last for 4 more days. This day the people who observe fast take bath at a river or pond and prepare lunch (consisting of rice, dal mixed with pumpkin, made in pure ghee).
The second day (5th day from Tihar) is known as Kharna or kheer- roti. In which the kheer ( A recipe where rice is prepared with sweetened milk instead of water) and chapati ( called roti ). The people observe fast for the full day without taking even water and eat this kheer-roti as dinner after offering it to the rising moon and Goddess Ganga. This is the only time when they eat or drink anything from the starting of the day till the last day of Chhath.
The third day is the main festival day (exactly 6th day from Tihar) of Chhath. The devotees maintain 'Nirjal Barta’ (Fast without even taking a drop of water ) on the third day. It mainly consist of going on river bank and offering 'Argha' ( offering of fruits and sweets in winnow ) and Surya Namaskar to the setting sun followed by the next day (exactly 7th day from Tihar) event of offering Argha and Surya Namaskar to the rising sun on the fourth or last day of Chhath. The fast is then come to end after offering Argha to rising sun. In this way, nearly 42 hours of strict penance comes to an end.
The rituals of the festival are rigorous and are observed over a period of four days. They include holy bathing, fasting and abstaining from drinking water (Vratta), standing in water for long periods of time, and offering Prasad (prayer offerings) and Arghya to the setting and rising sun. Some devotees also perform a prostration march as they head for the river banks.
Environmentalists claim that Chhath is the most eco-friendly Hindu festival. Although the festival is observed most elaborately in Madhesh (southern) region of Nepal.
Chhath puja is performed on Kartika Shukla Shashthi, which is the sixth day of the month of Kartika in the Vikram Samvat. This falls typically in the month of October or November in the Gregorian English Calendar. It is also celebrated in the month of Chaitra.
Chhath puja is on the 13th (sunset) & 14th (sunrise) of November 2018. The four-day festival will start from 11 November and will end on 14 November.
It is also celebrated in the summer (March–April), on Chaitra Shashthi, some days after Holi; this event is called Chaiti Chhath. Chhath is an arduous observance, requiring the worshipers to fast without sip of water for around 36 hours continuously.