Hindu calendar 2026 and 2027 हिंदू कैलेंडर

From the sky: the tithi is the Moon's distance from the Sun, and each vrat is kept on the day its tithi is current at the moment the tradition judges it, sunrise for most, sunset for Pradosh, moonrise for Sankashti, all for the city you choose.

Usually because of the city, since sunrise decides the day, or the tradition: Vaishnava calendars judge Ekadashi from before sunrise. North Indian calendars also name the dark half of a month one month later than South Indian ones; the day itself is the same.