Why your festival is on a different day

1. A tithi is not a day. A tithi is twelve degrees of the Moon's lead over the Sun. It begins and ends at an instant, the same instant everywhere, and it runs anywhere from about 19 to about 26 hours because the Moon does not move at a constant speed. So a tithi does not line up with a civil day, and something has to decide which day it belongs to.

2. What decides is a local moment. Usually sunrise: the tithi running at sunrise owns the day. Sunrise is yours alone. It depends on your latitude, your longitude and the height of your horizon. Two cities get the same tithi at different points in their own morning, and sometimes on different mornings.

3. Some festivals do not use sunrise at all. This is the step most explanations skip. Diwali's rule is that Amavasya must be running at pradosha, the first three muhurtas after sunset. Dhanteras and Holika Dahan are nightfall rules too. Shivaratri and Janmashtami need their tithi at nishita, the exact middle of the night. Ganesh Chaturthi and Rama Navami need midday. Vijayadashami needs the afternoon. Each of these is a different local moment, so each has its own way of landing on a different day in a different city.

4. Diwali is the clearest case. In 2025 the Amavasya began at 15:44 in India and it covered the sunset of 20 October in Chennai, in Delhi, in London and in New Jersey, so the whole world kept it on the 20th. In 2027 the same rule splits: Chennai keeps 29 October, New Jersey keeps the 28th. The tithi began at the identical instant for both. What differed was whose sunset it covered. And notice what Diwali does not require: on that Chennai morning in 2025 the tithi at sunrise was Chaturdashi, not Amavasya at all.

5. Then there is the month, and the month has two names. The south counts a lunar month new moon to new moon, the north full moon to full moon. For the bright half both agree. For the dark half they differ by one month, which is why the same night of Diwali is Ashvina Amavasya in the south and Kartika Amavasya in the north. The date is the same. The name is not. This page names months the southern way and prints the northern name beside it.

Computed, and testable. The Sun's and Moon's places come from a pinned ephemeris, the tithi boundaries are found by bisection to the second, the lunar month is bounded by two new moons, and the month is named by the sankranti inside it. Sunrise, sunset, the five fifths of the day and the fifteen muhurtas of the night all come from your coordinates. None of that is a matter of opinion and all of it is in the solver.

Picked, and therefore marked. Which local moment a given festival's rule points at is convention. Panchangams differ, regions differ, and two traditions can keep the same festival a day apart on purpose. Every festival above shows its rule. The ones known to be disputed carry a note saying so, and Janmashtami says plainly that the Smarta and the Vaishnava reckonings often land a day apart and that one date here cannot be right for everybody.

Honest note. The month naming rule cannot be derived; it is pinned in the solver against thirty published festival dates from 2025 and 2026, and if it were wrong by one month every date here would be wrong by a month and would still look plausible. Where a tithi is so short that it misses the window a rule points at, this page keeps the day whose night, or whose daylight, the tithi covers most of, and says on that festival that it had to. Regional festivals are missing: this is a short list, not a complete almanac, and it does not yet carry Onam, Pongal beyond the sankranti itself, Chhath, Bihu, Navaratri day by day, or the many local temple days. Dates before 1900 or after 2100 are not checked. The taste call on which festivals belong here, and the final word on every rule, is Hanu's.