Which day is your fast, actually

A tithi is a lunar day. It is not a day of the clock. It is a stage in the widening gap between the moon and the sun, and it ends once that gap has grown by another 12 degrees. The moon speeds up and slows down along its orbit, so a tithi comes out a bit longer or a bit shorter than one of our days. It will not sit neatly inside a date. It starts on one evening and finishes on the next.

Most Indian fasts, festivals and traditional birthdays are pinned to a tithi, not to a date. So the almanac needs a rule for deciding which of our dates carries which tithi's name. Here it is. Look at the moment of sunrise. Whichever tithi is running then gives the whole day its name, until the next sunrise. That is the udaya tithi, the risen tithi.

Watch it work. In August this year, on sheets computed for Delhi, the eighth lunar day, ashtami, begins at 19:19 on Wednesday the 19th and ends at 21:18 on Thursday the 20th. Sunrise is just before six on both mornings. At Wednesday's sunrise the eighth has not started, so Wednesday is the seventh. At Thursday's sunrise it is running, so Thursday is the eighth, and the fast goes there.

This is why a family argues. Somebody checks a phone on Wednesday evening, sees that the eighth has started, and assumes the fast is Wednesday. The phone was right. The tithi simply started too late in the day to claim it.

The same rule fixes a janma tithi birthday, the birthday kept by the lunar day somebody was born under rather than by the calendar date.

Two more things cause the same argument. Sunrise is local, so a sheet printed for another city is not quite your sheet. And some observances are decided by midday, or by the hour after sunset, or by midnight, rather than by sunrise.