2011
Monday, 31 October 2011
In 2011 the tithi runs from 11:03 am on 31 Oct to 10:06 am on 1 Nov; it is running at pradosh, the twilight just after sunset on Monday, 31 October 2011, so that is the day of Chhath Puja.
2012
Sunday, 18 November 2012
In 2012 the tithi runs from 11:24 am on 18 Nov to 9:31 am on 19 Nov; it is running at pradosh, the twilight just after sunset on Sunday, 18 November 2012, 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.