What are the four days of Chhath Puja and their significance?

Chhath Puja is a four-day festival dedicated to the Sun God to express gratitude for life and prosperity. Day 1, Nahay Khay, involves cooking bottle gourd with rock salt. On Day 2, Kharna, devotees break their day-long fast with jaggery-based kheer. Days 3 and 4, Sandhya Arghya and Usha Arghya, involve offering prayers to the setting and rising Sun, respectively.