Chatt is a key festival celebrated in Bihar, Jharkhand and the Terai regions of Nepal, India. Chhath Puja is performed to thank Surya (Sun) for sustaining life on earth. The devotees believe that their wishes are always granted. The word Chhath denotes the number 6 in Hindi and Chatt festival is celebrated on the sixth day of the Hindu lunar month of Kartik hence the name 'Chatt'.  Here's a small Video of Chhat Puja.