What is a tithi? Why Hindu festival dates move every year

Diwali was on 8 November in 2026, 20 October in 2025 and 1 November in 2024. Nothing about the festival changed; the calendar it is fixed in is not the one on your wall. Hindu festivals and fasts are fixed by tithi, the lunar day, and once you know what a tithi is, the moving dates stop being mysterious. The panchang shows today’s tithi for your city with the time it ends; this guide explains what you are looking at.

The Moon’s distance from the Sun

Watch the Moon over a month and it goes from invisible at new moon, through a thin crescent, to full, and back. What is changing is its angular distance from the Sun as seen from Earth — its elongation. At new moon the Moon is beside the Sun, elongation 0°. At full moon it is opposite, 180°. Then it closes the gap again until the next new moon at 360°, which is 0° once more.

A tithi is one thirtieth of that cycle: the time the Moon takes to move 12° further from the Sun. Thirty tithis take the Moon all the way round, and thirty tithis make a lunar month.

Thirty tithis, two halves

The first fifteen, from new moon to full, are Shukla paksha — the bright, waxing fortnight. The next fifteen, from full back to new, are Krishna paksha — the dark, waning one. Each fortnight numbers its tithis the same way: Pratipada, Dwitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi, Chaturdashi, and then the fifteenth is Purnima (full moon) in the bright half and Amavasya (new moon) in the dark half.

So “Ekadashi” on its own is ambiguous: there is a Shukla Ekadashi and a Krishna Ekadashi every month, which is why Ekadashi fasts come about twice a month. Festivals are named with the paksha and the month: Kartika Krishna Amavasya is Diwali, Bhadrapada Shukla Chaturthi is Ganesh Chaturthi.

Why a tithi is not a day

The Moon does not move at a constant speed — its orbit is an ellipse — and the Sun is moving too, about a degree a day in the same direction. So the 12° of elongation takes anywhere from about 19 to 26 hours, averaging 23 hours 37 minutes. A tithi begins and ends at odd times: 02:09, 14:48, 19:14.

The tradition’s rule for turning that into calendar days is the sunrise rule: a day is named for the tithi running at sunrise. If Ekadashi began at 3 a.m. and ends at 11 p.m., the day is Ekadashi. If it begins at 7 a.m., an hour after sunrise, today is still Dashami and tomorrow is Ekadashi — even though Ekadashi was running for seventeen of today’s hours.

Two consequences follow. A long tithi can be running at two consecutive sunrises, and the same date appears twice (adhika tithi). A short one can begin after one sunrise and end before the next, and its date never appears at all (kshaya tithi). Both happen every few weeks, and both are correct.

Why sunrise, and why your city

Since the day’s name depends on what is running at sunrise, it depends on when sunrise is — and that depends on where you are. Sunrise in Kolkata is about an hour before sunrise in Ahmedabad. A tithi that begins at 6:20 a.m. Indian time has started before Ahmedabad’s sunrise but after Kolkata’s, and the two cities name the day differently. Printed panchangs are made for one city; the panchang tool computes sunrise for yours, which is the only way to be right everywhere.

Why the dates move

Twelve lunar months of 29.53 days come to 354 days, eleven short of the solar year. So every festival, fixed by tithi, lands about eleven days earlier each year — until the gap grows to a month, and the calendar inserts an extra month (adhik masa, roughly every 32 months) to pull the festivals back into their season. That is why Diwali wanders between mid-October and mid-November and never further.

The tool shows the tithi with its end time, the paksha, the month in both the North Indian and South Indian reckoning, and the Ekadashi, Purnima and Amavasya dates for the next year, each computed at your own sunrise. For how a fast’s date is chosen when the tithi straddles two days, there is a separate guide.