Just after Diwali, on the forth day of the Shukla Paksha in Hindu month of Kartik this festival begins with Nahay Khay. A person performs the puja, observes fast, generally sleep on the floor and not on a bed or mattress. On this day, devotees wake up early, take a bath and consume food after that especially prepared for them by them. The fast continues till the person offers is/her prayers to the rising Sun.
The Puja revolves around the sun so the timings of sunrise and sunset are important. The main day is Shashthi of Shukla Paksha of Kartik month. On this day women offer Sandhya Arghya and observe a day-long fast and break it only on the following day after sunrise.
There are various stories associated with the origin of Chhath Puja. According to a legend, Pandavas and their wife, Draupadi used to offer prayers to Lord sun to get their lost kingdoms back and solve any problem. It is believed that since then, it became a mandatory custom to perform Chhath Puja.
Some other legends believe the inception of Chhath Puja is associated with Karna who was the son of Lord Surya and Mata Kunti. He was the ruler of Ang Desh, which is currently known as Bhagalpur in Bihar. It is said that Karna used to perform Chhath Puja during his time.