I was migrating our regional calendar pages from hand-coded festival dates to engine-computed ones when I noticed Bhai Dooj 2026 was showing November 11. I checked it against a couple of published panchangs. Both said November 10.
The festival was off by a day. The fix turned out to be two lines, and those two lines are agreeing with a body of seventeenth-century Sanskrit law that said this question would come up and answered it in advance.
A tithi is not a day.
A tithi is the duration during which the Moon moves twelve degrees of longitude away from the Sun. The Moon's orbit is elliptical, so the duration isn't constant. It varies between about twenty hours at the fastest and twenty-seven at the slowest. The average is twenty-three hours and thirty-seven minutes. Almost never a clean multiple of the solar day.
So a tithi spans, on most days, two Gregorian dates. It begins partway through one day, runs across the night, ends partway through the next.
