Biggest festival of Bihar!

Chhath Puja in Bihar uniquely worships both the setting and rising sun.
Celebrated on the sixth day of Kartik month, it is also called Surya Shashti.
Chhath Puja is celebrated for four days. Day one, Nahay Khay includes a holy water dip, house cleaning and a simple meal.
The second day, Kharna, involves a day-long fast, broken with offerings of rice pudding and fruits.
The third day is dedicated to preparing offerings for the evening ritual, Sanjhiya Arghya. It also features the colourful Kosi event with sugarcane canopies and lit lamps.
Folk songs and cultural displays are prominent.
On the final day, devotees make sunrise offerings to the rising sun, break their fast, and share Prasad with neighbours and relatives.
During Chhath Puja, devotees gather at riverbanks, offering prayers to the setting sun.

Resources:
https://tourism.bihar.gov.in/: Chhath Puja
NDTV: Chhath Puja 2023: Celebrations And Significance Of Festival: 14 Nov 2023
Hindustan Times: Chhath Puja 2022: Why is Chhath Puja celebrated? Know date, history, significance, celebrations of four days of Chhath: 28 Oct 2022