Apparently some old inscriptions have incorrect weekdays; see for example https://x.com/hamsanandi/status/1906223551545512152 where a historical date corresponds to (what we’d now call) Saturday, but the inscription says Monday (sōmavāra). One possibility (and the correct one as I believe—see below) is that this is just a mistake in the inscription, but another hypothesis one could conceive of is that weekdays were different then, i.e. there was at some point a “reset” that brought them in sync with the present-day cycle of weekdays.
On the one hand, this hypothesis is not inconceivable, as unlike tithi or nakṣatra there is no astronomical “ground truth” for weekdays (that I know of); the days of the week simply cycle through the 7 possibilities by convention. (Yes I know of the 24-hora system—see also names of the days of the week and planetary hours—but that only establishes the relative order, not the offset.)
So it is in principle possible that at some point the weekdays were “reset”, say someone declared a day to be Sunday (even if the previous day were not Saturday), and then the weekdays followed this order from then on, somewhat like the Gregorian calendar reform:
The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).
On the other hand, I find this hypothesis unlikely, because:
- the days of the week are something people can simply count/advance one day at a time, and requires even less work to keep track of than tithi and nakṣatra (though as mentioned these have a “ground truth” that is to some extent discernible to the naked eye), and it would be difficult to get everyone to change,
- if jyotiṣa (the astrology part) is to be believed, the days (like tithi-s and nakṣatra-s) have astrological significance, e.g. Sunday really has certain features of the sun about it: certain days of the week are more conducive to certain actions/prayers (the corresponding gods are more easily accessible), so this would imply the usage of “incorrect” days either before or later without anyone noticing it (which again, is unfortunately not inconceivable).
So anyway, I got AI to look into this hypothesis (whether the weekdays have shifted over the centuries) for a bit, and the data is gathered here: https://shreevatsa.net/tmp/2026-10/indian-inscriptions-weekdays/
What the data seems to suggest (needs more verification) is that across the centuries, the majority (75–80%) of inscriptions are more consistent with the current weekdays than any other. That is, a reset seems unlikely; the days have been in sync.
Taking this as established (I’m convinced by the data, though again I’m biased against a “shift”/“reset” hypothesis), there still remains the question of why there are so many discrepancies. After all, 75–80% is very small compared to 100%; how could inscriptions list incorrect weekdays once every four or five times? A delta of one day in either direction is explainable by the fact that tithi/nakṣatra boundaries don’t match sunrise/sunset, but that still leaves ~11.2% of inscriptions that differ by two or three days.
My current guesses are:
- That these mistakes happened because weekdays were simply not a big part of life. (That they were not is true: yes, certain pūjā-s would happen on certain days, but otherwise we did not have the present-day “weekday-weekend” system with a specific day like Sunday reserved for rest, etc.) Even today in traditional pāṭhaśālā-s etc, breaks (anadhyayana) are decided based on tithi, not on the day of the week. So maybe the inscriptions made mistakes at such a high rate because they were relying on memory and it’s easy to make mistakes. More research is needed (including verifying the data!)
- It is possible that in many of the inscriptions currently treated as not matching, the weekdays are correct, but the other elements—year, month (māsa), day (tithi/nakṣatra)—are incorrect. (Which can easily happen if the systems of timekeeping—vākya/siddhānta etc—were not accurate.) So we should go over the list focusing on dates that can be independently dated (days of eclipses etc).
Link to the data so far: https://shreevatsa.net/tmp/2026-10/indian-inscriptions-weekdays/