The Indian festival calendar — how the dates are worked out

The short version

Most Indian festivals are set by the moon, and the moon does not keep step with the Gregorian calendar. A festival that falls in early November one year falls in late October the next, drifts back, and then jumps forward when an extra month is inserted to stop the seasons sliding.

That is why the date changes. What follows is why two calendars can disagree even in the same year.

The tithi is not a day

The lunar day, the tithi, is defined by the angle between the sun and the moon — each tithi is the time the moon takes to gain twelve degrees on the sun. That takes anywhere from about nineteen to about twenty-six hours, and it begins whenever it begins, not at midnight.

So a tithi can start at four in the afternoon on Tuesday and end at one in the afternoon on Wednesday. Which of those two days "is" the festival depends on rules — usually about which day the tithi is current at sunrise, or at midnight, or at the specific hour the observance requires.

Two consequences follow, and between them they explain nearly every calendar disagreement:

• Location changes the answer. Sunrise in Chennai and sunrise in New Jersey are hours apart, so the tithi at sunrise can fall on different Gregorian days.
• Tradition changes the answer. Where an observance depends on a particular hour — midnight for Janmashtami, midnight for Shivaratri, which is worshipped through the night in four watches but dated by the tithi at nishita — different traditions apply different rules when the tithi does not cover that hour cleanly.

The two month systems

Both are correct, and both are in daily use.

The festivals fall on the same nights in both. Only the month names attached to them differ, which is why the same festival is described as belonging to different months in different books.

And they only differ for half of each month. Through the waxing fortnight, and on the full moon itself, both systems give the same month name — so Ganesh Chaturthi is Bhadrapada everywhere, and Holika Dahan is Phalguna everywhere. The two names separate the moment the moon starts waning, because that is when a purnimanta month has already rolled over and an amanta one has not. Every festival that needs the two-name warning is a waning-fortnight festival: Janmashtami is Bhadrapada in the north and Shravana in the south, Maha Shivaratri is Phalguna in the north and Magha in the south, and Holi's colour day is Chaitra in the north and Phalguna in the south.

The solar calendars

Several regions run solar calendars alongside the lunar one, and these set the regional new year:

• Tamil Nadu — Puthandu, at the sun's entry into Mesha
• Kerala — Vishu, on the same solar event, with the Kollavarsham year counted separately
• Bengal and Assam — Poila Boishakh and Bohag Bihu, in the same mid-April window
• Punjab — Vaisakhi, likewise, and a harvest festival as well as a calendar marker

This is why mid-April carries a cluster of new years across otherwise unrelated regions: they are all keyed to the same solar moment.

How to find a date you can trust

1. Use a computation made for your location, not for India, if you live elsewhere. Several established panchang services publish city-specific dates.
2. Check what your temple or your family follows, particularly for Janmashtami and Shivaratri, where traditions genuinely diverge and both dates are legitimate.
3. For anything with a muhurta — Holika Dahan, Ghatasthapana, Lakshmi Puja — the hour matters as much as the day, and only a panchang gives you the window.
4. Expect two dates in some years. It is not an error, and picking the one your household has always followed is the correct resolution.