Hindu calendar 2021

Why your date may differ

A Hindu festival is fixed to a tithi — a lunar day — not to a calendar date. A tithi begins and ends at the same instant everywhere on Earth, but local sunrise does not. So the tithi that governs a festival can be running at sunrise in Mumbai and already over by sunrise in California, and the same festival honestly falls on two different calendar days in the two places. That is why these dates are published for five regions rather than one, and why a family calendar and a temple calendar elsewhere can disagree by a day without either being wrong.

How these dates are computed

Sources

Positions of the Sun and Moon come from Astronomy Engine, an open-source implementation of standard astronomical models.

Tithi and nakshatra

Tithi is the Moon’s elongation from the Sun in twelve-degree steps. Nakshatra uses the Lahiri ayanamsa. The times given against each date are that tithi’s own beginning and end — the same two instants everywhere on Earth, shown on the clock of the region selected above, which is why they can read as a different day in California from the day they read in Ujjain.

Months

Months are calculated amanta — new moon to new moon, named for the solar month whose sankranti falls within them — while festival names follow the purnimanta convention every published almanac prints, which is why a bright-fortnight festival carries the name of the month after the one its lunation belongs to. An intercalary adhika month carries no festivals.

What is verified

A hand-checked table of dates from published sources — every festival rule across at least three years, two full years of Ekadashi, and a full year of Purnima and Amavasya — is re-verified on every build, and the build is refused if any disagree. That table is a sample, not a copy of everything below it: the rest of these dates follow from the same rules it holds to account, rather than having each been checked one by one.

Uneven tithis

A tithi is a lunar day of uneven length, so now and then one begins and ends between two sunrises and is never running at the instant its rule names. About one date in twenty here is such a case, and falls back to the civil day the tithi itself runs through. Those dates carry a line saying so, both in the list above and in the calendar feed.

Not included

Two things this calendar does not claim to settle. Holi is not listed: its date depends on when Bhadra clears the Holika Dahan window, and we could not reproduce that rule from sources closely enough to publish it. Akshaya Tritiya is not listed either, for the same reason — its observance could not be reproduced from a single reference instant across the years these feeds cover. In both cases, an absent date seemed better than a confident wrong one. And where Ashtami and Rohini fall on different nights, traditions differ on which carries Janmashtami; these dates follow the Smarta reckoning, on the Ashtami prevailing at nishita, the middle of the night.

Ekadashi

Ekadashi is reckoned the Smarta way throughout: the Ekadashi tithi prevailing at sunrise, with the fast broken the following day during Dwadashi. Vaishnava practice sometimes keeps the fast a day later than the date given here. Only one reckoning is published rather than both, so if your tradition is the Vaishnava one, read these as the Smarta dates and adjust as your temple does.

Corrections

If a date here is corrected, subscribers get the correction. Each entry in the feed carries a revision number that goes up when its content changes, so a calendar app takes the newer version rather than keeping the copy it already had. Nothing that has not changed is republished, so a redeploy never arrives as a pile of edits.