Chhath is a very ancient Hindu Vedic festival. The festivities are dedicated to Hindu God Sun and Chhathi Maiya (Vedic Goddess). It is celebrated mostly in Bihar and some other Northern states of India and madhesi state of Nepal.
Chhath pooja is observed to thank Hindu god Sun that gives energy to sustain life on earth. In Hindu religion, it is believed that worshipping God Sun will help cure certain diseases that include leprosy. Surya ( God Sun) is also worshipped for attaining blessings of longevity and prosperity for the family members.
Chhathi Maiya: Chhathi Maiya is called Usha in Vedas and is believed to be younger sister of sun God. The festival celebrates rising sun as well as setting sun.
The four days of the festival
On the first day or Nahay Khay, devotees take a dip in Kosi river, Karnali or Ganga and take some water to prepare offerings.
Lohanda and Kharna: Woman devotees ( called vratins) observe fast for the whole day and resume it after worshipping sun and Moon. Prashad is distributed to family members and friends and after prashad the fast is continued by Vratins for 36 hours.
Sandhya Arghya: On the third day, prashad is prepared and vratins along with family members go to apond or any large water body where they worship the setting sun.
Usha Arghya: On the final day, offerings are made to the rising sun and the fast is finally resumed.