When is Guru Purnima?

How the date is decided

Guru Purnima is Ashadha Shukla Purnima — the full moon of Ashadha, the fifteenth tithi of the waxing fortnight — judged at sunrise.

Sunrise is the default reference across this calendar and the one most vrats use. The day whose sunrise falls under Purnima is the day that carries the festival.

The same full moon, a different day

Here is the part worth reading, because it is the cleanest demonstration on this site of why the reference moment is not a technicality.

Satyanarayan Vrat is observed on every Purnima, including this one. It is therefore the same tithi in the same lunar month as Guru Purnima. And it can fall on a different civil date, because it is judged at madhyahna — solar midday — rather than at sunrise.

Picture a Purnima that begins at 09:00 and ends at 07:00 the following morning:

• sunrise on the first day is under Chaturdashi; on the second it is under Purnima, so Guru Purnima takes the second day
• midday on the first day is under Purnima; on the second it is not, so Satyanarayan takes the first day

Two observances, one tithi, two dates — with no disagreement anywhere about the astronomy. Every published festival-date dispute you will encounter has this shape underneath it.

Why Satyanarayan uses midday

The obvious reading is that the puja is performed in the evening, so the reference should be after sunset. That reading was tested against a full year of independently published dates and it failed: an evening reference reproduced 10 of 13 dates and a sunrise reference only 8, while midday reproduced all 13, and then reproduced a second, held-back year in full without further adjustment.

Midday is the moment that identifies the day carrying the bulk of the tithi, which is what a day-long fast broken in the evening actually needs. The result is unintuitive and it is what the evidence supports.

Satyanarayan is also unusual in firing inside an Adhika (intercalary) month, where major festivals are suppressed — a leap month has a Purnima of its own, and the vrat follows the full moon rather than the festival calendar.

Why the Gregorian date moves

Ashadha Purnima falls in July, occasionally very early August. The lunar year runs about eleven days short of the solar one, so the date drifts earlier annually until an intercalary month resets it. Because major festivals do not fire in an Adhika month, a leap Ashadha moves Guru Purnima to the true Ashadha — while Satyanarayan, as above, still fires in the leap month.

What we do not compute

We fix the day. We do not compute the puja muhurat, the guru-specific observances kept by individual sampradayas, or the Buddhist observance of the same date. For choosing an hour, the choghadiya and the full panchang give the day’s windows for your own city.