THE BLOG · EXPLAINER
why hindu festival dates move every year
diwali is not on the same date every year, for the same reason easter is not. it follows the moon, not the sun. here is what a tithi is, and why ur family's calendar might not match ur phone.
every year someone asks why diwali "moved". it did not move. the calendar on ur phone is simply not the calendar diwali is booked on.
diwali, holi, navratri and most big hindu festivals are set by tithi, which is a day in the moon's calendar. they are not set by a fixed date in the sun's calendar.
once u see how a tithi works, the dates stop looking random. they are a second clock, older than the one on ur phone, and it is running perfectly.
THE QUICK CHECK
check any date's tithi
pick a date. we tell u the tithi and paksha running that day. no birth details needed. this one is the same for everybody.
This is a quick check, computed for noon IST on the chosen date. Tithi can change mid-day, so a date right at a tithi boundary may read differently depending on the exact hour.
the calendar u're actually using without knowing it
most of the world runs on the gregorian calendar. it is a sun calendar. about 365 days, one trip of the earth around the sun, and the dates land on the same day every year almost by design. that is what ur phone shows.
hindu festivals run on something else: the panchang. it follows the moon, and gets nudged now and then to stay in step with the sun.
and its "day" is not a fixed 24 hours. its day is a tithi, and a tithi is measured by angle, not by the clock.
what a tithi actually is
picture the sun and the moon on a giant clock face. a tithi is however long the moon takes to pull 12 degrees further away from the sun. that is the whole definition.
a full moon month holds 30 of them. fifteen while the moon is filling up, called shukla paksha, from new moon to full moon. fifteen while it empties out, called krishna paksha, from full moon back to new moon.
here is the part nobody mentions. the moon does not travel at a steady speed, so tithis are not all the same length. one can run about 19 hours, another about 26.
that means a moon day and a sun day never quite line up. a tithi can even get skipped, or counted twice, inside a single ordinary day, depending on where the sun and moon stood at sunrise.
why that makes festival dates shift on the solar calendar
a sun year is about 365.24 days. twelve moon months add up to about 354. so the moon calendar is roughly 11 days short every single year.
that shortfall piles up. a festival tied to a tithi creeps earlier and earlier against ur phone's calendar. so every two or three years the panchang bolts on a whole extra month, called adhik maas, to drag it back into place.
every lunar calendar has to answer this problem somehow. the gregorian one ignores the moon completely. islamic calendars let the dates drift and never correct them, which is why ramadan slides through the seasons. the hindu panchang corrects now and then, so festivals stay in roughly the same season. three answers, one mismatch.
amanta vs purnimanta: two ways to name the same month
there is a second thing going on, and it is regional. people disagree about where a moon month starts and stops.
amanta ("ending at amavasya"): the month runs new moon to new moon. used across most of south india, maharashtra, gujarat, and bengal.
purnimanta ("ending at purnima"): the month runs full moon to full moon. traditionally used across much of north india.
the tithi itself is the same everywhere in india on any given day. what changes is which month u file it under.
take the waning half that comes right after a full moon. under amanta it is the tail of one month. under purnimanta it is the start of the next one. same sky, same tithi, different label on the box.
that is why a north indian panchang and a south indian panchang can call a festival's month by different names while agreeing on the actual day. same system, different idea of where the month ends.
the receipt
the 12-degree tithi, the 30-tithi month, the 354-day moon year and the amanta versus purnimanta split are all standard, well documented parts of the panchang. we have not simplified any of it.
and the uneven tithi length, sometimes under 20 hours, sometimes over 26, is not a rounding error. the moon's path is an oval and its speed changes, so the tithis have to come out uneven.
Ask Tota works out tithi from the same sun and moon positions it uses everywhere else. one nice thing about tithi: because it is just the gap between the sun and moon, the ayanamsa cancels out. so western and vedic astrologers agree on today's tithi even when they argue about today's rashi.
faq
why don't hindu festivals fall on the same date every year?
because they are dated by tithi, which is a day in the moon's calendar, not a date on the 365-day sun calendar ur phone uses.
what is a tithi?
the time the moon takes to pull another 12 degrees away from the sun. there are 30 in a moon month, and each one runs somewhere between about 19 and 26 hours.
what's the difference between amanta and purnimanta?
both are ways of counting moon months. amanta months run new moon to new moon, and are used in south india, maharashtra, gujarat and bengal. purnimanta months run full moon to full moon, and are used across much of north india.
why do lunar calendars need a leap month?
twelve moon months come to about 354 days, roughly 11 short of a sun year. so an extra month called adhik maas gets added every two or three years, to keep festivals in the same season.
does tithi depend on which ayanamsa an astrologer uses?
no. tithi is just the gap between the sun and the moon, and ayanamsa moves both of them by the same amount, so it cancels out. western and vedic astrologers agree on today's tithi.