Chhath Puja is a four-day festival of thanks to the Sun and to Chhathi Maiya, kept mainly in Bihar, eastern Uttar Pradesh, Jharkhand and Nepal, and by communities from these regions around the world. The main day is Kartika Shukla Shashthi, the sixth day of the bright half of the month of Kartika, a few days after Diwali. Devotees fast and stand in a river, pond or tank to offer arghya, water and offerings, to the setting Sun on that evening and to the rising Sun on the next morning.
The day is the civil day on which Shashthi is in force at sunrise, and the evening offering is made at the sunset of that day. The table therefore gives the sunrise and the sunset of that day for each city. The morning offering is at the sunrise of the next day, a little later than the sunrise shown. Families usually begin the fast with the nahay khay on the day before and end with the parana on the morning after.
Why do cities differ? The times of sunset and sunrise depend on latitude, longitude and the season, and in November they change quickly. A city far to the north has a much earlier sunset than one near the equator. The tithi may be running at sunrise in one city and have ended before sunrise in another, which in some years moves the day.
A calm note: Chhath is a demanding fast, kept without water for long hours in some families. If you have a health condition, are pregnant or elderly, speak to a doctor about how to take part. Take care near water, especially with children, and follow local rules for gatherings at rivers and lakes. If your family keeps a different day, follow them.