Mostly concentrated in the areas of Bihar and Nepal, Chhath Puja is an important Hindu festival. In this, huge crowds gather to worship the Sun God and his wife, Usha. The divine, in its rawest form (nature), is hailed and thanked for supporting life on earth. Chhath Puja spans over four days and is celebrated on the sixth day of the Kartika month (October-November) in the Hindu calendar.

How is it celebrated?
The four-day-long Chhath Puja starts four days after Diwali. The festivities and rituals are as follows:
First Day (Nahay Khay): Devotees take a dip in the Kosi, Ganga, Karnali (Nepal) rivers or any nearby waterbody. The holy water collected from the river is then carried home.
Second Day (Lohanda): Devotees observe a day-long fast, which ends after sunset. After offering prayers to the sun and the moon, they share a meal of rice, bananas and kheer (rice pudding) with family and friends. After this, a 36-hour fast is observed.
Third Day (Sandhya Arghya): After the holy offering (prasad) is prepared, devotees take a dip in a river/ waterbody and worship the Sun God and Chhathi maiyya. They offer evening prasad and celebrate amid folk songs. 
Fourth Day (Usha Arghya):  In the morning, devotees head to the nearest water body and offer morning prasad to the sun, after which they break their fast. 
During the period of Chhath Puja, devotees follow a simple lifestyle: they live frugally, sleep on the floor and use a single blanket. The third day of the puja is believed to be most auspicious when Surya Namaskar (sun salutation) and fruits are offered to the Sun God. 

Why is it celebrated?
Legend has it that a long time ago, in the month of Kartika, when Lord Rama and Goddess Sita returned to Ayodhya after their exile, they fasted and offered prayers to the Sun God. Since then, it became traditional to celebrate the puja zealously every year.