Festival & Vrat Calendar: Dates Computed From the Tithi

Every year someone asks why Diwali is on a different date than last year. The answer is straightforward once you see it, and it explains almost everything else about this subject.

Hindu festivals are defined by tithi, not by calendar date.

What a tithi is

A tithi is a lunar day: the time the Moon takes to gain twelve degrees on the Sun. Thirty of them make a lunar month, fifteen waxing (shukla paksha) and fifteen waning (krishna paksha).

And every festival is a tithi, not a date:

• Diwali is Amavasya of Kartik - the new moon tithi of that lunar month.
• Holi is Purnima of Phalguna - the full moon.
• Janmashtami is Ashtami of Krishna paksha in Bhadrapada - the eighth tithi of the waning fortnight.
• Ekadashi is the eleventh tithi, which is why it comes round twice a lunar month.
• Ganesh Chaturthi is Chaturthi of shukla paksha in Bhadrapada - the fourth tithi.

The Gregorian date is simply wherever that tithi happens to land. Nobody chose it, and it moves because the tithi moves.

The eleven-day gap

Twelve lunar months come to about 354 days. A solar year is about 365. The lunar year is therefore roughly eleven days shorter.

So each year a festival falls about eleven days earlier against the Gregorian calendar than it did the year before. Diwali in late October one year, mid-October the next.

Left alone, that drift would keep going. In thirty-three years the gap would be a full year and Diwali would arrive in summer.

Adhik maas - the correction

So roughly every third year, an extra lunar month is inserted. It is called adhik maas, also mala maas or purushottam maas.

It is a genuine calendrical correction, and the principle is the same as a leap year solving a different problem - a calendar built on one cycle being reconciled with another. Without it, the entire festival calendar would drift through the seasons.

The rule for which month gets doubled is not arbitrary either: an adhik maas occurs when a lunar month contains no solar ingress - no sankranti. That month is then repeated.

Now the part that causes real confusion

A tithi does not start at midnight and rarely lasts exactly one day.

Because the Moon's orbit is elliptical, its speed varies, and a tithi can run anywhere from about nineteen to twenty-six hours. So a tithi can begin one afternoon and end the next.

Which day gets the festival?

The usual rule is the tithi running at sunrise. But several festivals have their own rules - some go by the tithi at moonrise, some at a specific time of night, some require the tithi to be present at a particular moment. The Ekadashi vrat has its own conventions entirely, and there are separate observances (smarta and vaishnava) that can fall on different days.

And since sunrise differs by location, two cities can genuinely observe on different days.

That is not an error in one of the calendars. It is the rule working correctly in two places. A tithi that changes at 6:15 am is running at sunrise in one city and not in another two hundred kilometres west - and both are following the same rule.

Amanta and purnimanta

There is a second regional difference, and it confuses people who move between regions.

Amanta reckoning ends a lunar month at the new moon. Purnimanta ends it at the full moon. Amanta is common in the south and west; purnimanta in the north.

The result is that the same day can sit in differently named months depending on where you are - the festival is the same, the tithi is the same, but a northerner and a southerner will name the month differently. Both conventions are internally consistent, and neither is a mistake.

We print both, because it is a common source of confusion and it costs nothing to clear up.

Why this page computes rather than copies

Most festival calendar pages reproduce a list made for one city - usually Delhi or Ujjain - and present it as the calendar.

That is fine most of the time and wrong some of the time, and the wrong times are exactly the ones people argue about. We compute the tithi for the coordinates you give, so the dates reflect your own sunrise.

And we print which tithi each festival is defined by, so you can see why the date falls where it does rather than taking it on trust.

Festival dates for your year

Each one with its Gregorian date, the defining tithi, the paksha and the lunar month - plus the moment that tithi begins and ends for your own location.

Why the dates move

The eleven-day gap between twelve lunar months and a solar year, how it accumulates, and where the festivals would end up without a correction.

Adhik maas, and the rule behind it

The extra month inserted roughly every third year, the rule that decides which month is doubled - a lunar month with no solar ingress - and when the next one falls.

When a tithi straddles two days

The sunrise rule, the festivals with rules of their own, and why two cities can legitimately observe on different days under the same rule.

Amanta and purnimanta

The two month-naming conventions, where each is used, and how they shift which month a festival is said to fall in without changing the festival at all.

Ekadashi and the monthly vrats

Twenty-four Ekadashis a year, Pradosh twice a month, Sankashti once - with the tithi behind each and why Ekadashi has conventions of its own.

Why this page computes rather than copies

What a copied almanac gets right, where it goes wrong, and why the wrong cases are exactly the ones families argue about.