Yamuna Chhath is mainly celebrated in Indian city Mathura. It marks the day when Goddess Yamuna descended on the Earth. Hence this day is also known as Yamuna Jayanti or birth anniversary of Goddess Yamuna. It is celebrated on Shukla Paksha Shashthi in Chaitra month and falls during Chaitra Navratri.
Goddess Yamuna, being wife of Lord Shri Krishna, is revered by people of Braj which explains Yamuna Chhath being celebrated mainly by people of Mathura and Vrindavan.