What is a tithi — and why does Diwali's date change every year?

Every year someone asks: why was Diwali on 20 October last year and 8 November this year? The answer is simple. Hindu festivals follow the Moon, not the calendar on your wall.

A tithi is a Moon-day. The Moon moves a little further from the Sun every day, and each 12° step is one tithi. Thirty tithis make one Hindu month, from one new moon to the next. But the Moon does not move at a steady speed. So a tithi can be as short as 19 hours or as long as 26, and it almost never ends at midnight. That is why a panchang gives an end time for every tithi — “Chaturdashi till 2:32 pm, then Amavasya” — instead of a date.

Diwali is the new-moon tithi of the month of Kartik. Twelve Hindu months are about 11 days shorter than one year, so Diwali comes a little earlier every year. Every few years an extra month (adhik maas) is added to bring it back. That is why the date keeps moving.

One last thing. The tithi that counts for a day is the one running at sunrise — and sunrise is different in every city. So Jyotisha works out today's tithi for your city, not for a default one.