2017
Wednesday, 25 October 2017
In 2017 the tithi runs from 9:38 am on 25 Oct to 12:15 pm on 26 Oct; it is running at pradosh, the twilight just after sunset on Wednesday, 25 October 2017, so that is the day of Chhath Puja.
2018
Tuesday, 13 November 2018
In 2018 the tithi runs from 1:51 am on 13 Nov to 4:22 am on 14 Nov; it is running at pradosh, the twilight just after sunset on Tuesday, 13 November 2018, 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.