How a festival date is fixed

A festival is not fixed to a date. It is fixed to a tithi, a lunar day, and a tithi rarely covers a whole civil day. So each festival also names the instant at which its tithi has to be running: sunrise for most of them, and midday, aparahna, sunset, pradosha, nishita or arunodaya for the rest. The tithi and the instant together pick the day.

That is the whole mechanism. Two almanacs that agree perfectly about the sky can still print different days for one festival, because they read the same tithi at different instants. When that happens here, the page says which instant it used.

A tithi is the Moon’s longitude minus the Sun’s, in steps of 12 degrees, so thirty of them fill a lunar month. It is an angle, not a stretch of daylight, and the Moon draws away from the Sun at an uneven rate. Measured over 2024 to 2028, a tithi here runs as short as about twenty hours and as long as about twenty-seven.

That has two consequences a printed calendar has to handle. A tithi can begin after one sunrise and end before the next, so no day holds it at sunrise at all; that is a kshaya tithi, a lost one, and its number is simply missing from the calendar. Or it can cover two sunrises, and two days in a row carry the same number.

Counting a festival by sunrise alone would therefore sometimes name no day and sometimes name two. Naming an instant is how the tradition settles it.

Each of these is computed from the day’s own sunrise and sunset at the place you choose, so each of them moves with the season and the town.

Sunrise is the udaya tithi, the commonest rule and the default. Midday is the midpoint of the daylight. Aparahna is the fourth of the five equal parts of the day: it runs from six tenths to eight tenths of the daylight after sunrise, and a rule that names it usually asks for a tithi that covers any part of that window rather than one that holds a single instant.

Sunset is sunset. Pradosha is the evening that follows it, to about 2 hours 24 minutes after. Nishita is true midnight, the midpoint between sunset and the next sunrise, which is not the same as midnight on the clock. Arunodaya is the 96 minutes before sunrise, the hour and a half in which the Deepavali oil bath is taken.

Most rules carry a fallback, because the first instant is not always met in a given year. Where a fallback is used, the printed rule says so.

Maha Shivaratri is the Magha Krishna Chaturdasi running at nishita, and at sunset when no night carries it. The observance is a night vigil, so the rule reads the middle of the night rather than the morning. Magha here is the lunar month holding the Kumbha sankranti, which makes this the Krishna Chaturdasi before the Maasi Amavasai.

Deepavali in the Tamil list is the Ashvina Krishna Chaturdasi running at arunodaya, and at sunrise when it fails. The rule reads the pre-dawn hour because the observance is the oil bath, and the oil bath is taken before sunrise. That usually puts Tamil Deepavali on the day before the Amavasai.

Vijayadasami shows what happens when two traditions read one tithi at two instants. The Tamil list takes the Ashvina Shukla Dasami running at aparahna, which is the classical aparahna-vyapini rule and the one the Tamil Nadu holiday list follows: 12 October in 2024 and 20 October in 2026, where reading the tithi at sunrise would give the day after. Kerala and Bengal keep the udaya Dasami, at sunrise, and are then a day later. Both are applying a stated rule correctly, and the difference is in the rule, not in the astronomy.

The same pattern runs through the year. Vinayagar Chaturthi is read at madhyahna, midday. Karwa Chauth is read at sunset, because the moon is looked for within about three hours of it. Dhanteras is read at pradosha. Each instant is the hour at which the thing is actually done.

A festival fixed by a tithi also needs a month to sit in, and twelve lunar months fall about eleven days short of a solar year. Left alone, the lunar calendar would walk backwards through the seasons until Deepavali landed in the monsoon.

The correction is an extra month. A lunar month is named for the sidereal sign the Sun enters inside it. About once every thirty-two or thirty-three months a lunar month holds no ingress at all, and that month is adhika, intercalary. It borrows the name of the month that follows and hosts no festival, so every rule falls in the nija, the true month, instead. Adhika Jyeshtha in 2026 is the current example.

On this site the festivals fixed by a tithi are placed in the amanta lunar month that contains the matching sankranti. Chaitra is the lunar month holding the Mesha sankranti; Bhadrapada is the one holding the Kanya sankranti. That placement is what keeps Vinayagar Chaturthi and Deepavali where they belong in an adhika year, rather than a month early.

The arithmetic is checked rather than asserted. Over every lunation from 1900 to 2100, the month name and the adhika and kshaya flags computed here agree with the Swiss Ephemeris on every one, and the count of adhika months over that span comes out between seventy and eighty.

Every festival entry on this site carries the sentence that produced its date: which tithi, in which lunar month, read at which instant, with the fallback named if one was used. It is printed so that you can set it beside your family’s almanac and find the exact point where the two part.

Most of the time that point is the instant, not the sky. Once you can see that one list read the Dasami at sunrise and the other at aparahna, there is nothing left to argue about, and the question becomes the ordinary one of which household you are keeping the day with.

Pournami, Amavasai, Ekadasi and Pradosham are generated once per lunar cycle and paksha rather than listed by hand, so they cannot go stale or run out at the end of a table someone typed.

What each festival means and when its puja hour falls.

Festivals →