तिथि (Tithi)

Tithi is a lunar day in the Hindu calendar — the time the Moon takes to move 12 degrees ahead of the Sun. There are 30 tithis in a lunar month: 15 in the waxing (Shukla) fortnight and 15 in the waning (Krishna) fortnight. Hindu festivals are timed by tithi rather than calendar date, which is why their dates in the Gregorian calendar shift each year.

A solar day is exactly 24 hours — the time the Earth takes to rotate. A tithi is the time the Moon moves 12° relative to the Sun, which varies from about 19 to 26 hours due to the Moon's elliptical orbit. Sometimes a tithi can be skipped (Kshaya tithi) or repeat (Vriddhi tithi) in the Gregorian calendar, which is why panchang consultation is needed for exact festival timings.