What it means
A tithi is not a calendar day. It is the interval during which the angular distance between the Moon and the Sun increases by exactly 12 degrees, so thirty tithis complete one lunar month - fifteen waxing (Shukla Paksha) and fifteen waning (Krishna Paksha).
How it is read
Because the Moon's speed varies, a tithi can run anywhere from about 19 to 26 hours. It therefore drifts against the clock: a tithi can begin mid-afternoon and end the following morning, and occasionally one is skipped or repeated across two sunrises.
What to watch for
Almost every Hindu festival is fixed by tithi rather than by date, which is why the same festival lands on a different Gregorian day each year. The convention for which day a festival belongs to - usually the tithi running at sunrise, but not always - is where regional calendars most often diverge.
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