2039
Sunday, 20 November 2039
In 2039 the tithi runs from 7:14 pm on 20 Nov to 5:01 pm on 21 Nov, and it never reaches pradosh, the twilight just after sunset, so the day on which it begins is taken: Sunday, 20 November 2039.
2040
Friday, 9 November 2040
In 2040 the tithi runs from 2:32 pm on 9 Nov to 12:13 pm on 10 Nov; it is running at pradosh, the twilight just after sunset on Friday, 9 November 2040, so that is the day of Chhath Puja.
How the date is fixed
Chhath Puja falls on the sixth day of the bright half of the lunar month Kartika (Shukla Shashthi). The date is the day on which that tithi is running at pradosh, the twilight just after sunset.
About
Chhath is kept on Kartika Shukla Shashthi, the sixth tithi after the Diwali new moon, in October or November. The four days run Chaturthi (nahay-khay), Panchami (kharna), Shashthi (the evening arghya to the setting Sun) and Saptami (the morning arghya to the rising Sun). The Shashthi evening is the main observance, so the day whose sunset carries Shashthi is chosen and the almanac prints the local sunset and the next sunrise. A smaller Chaitra Chhath is kept in the spring on Chaitra Shukla Shashthi.
How it is observed
A 36-hour fast without water after kharna, and arghya offered standing in water to the setting and then the rising Sun.