Purnima vrat date and Amavasya: why the fast can come a day early

Every Hindu calendar marks two days a month above the rest: the full Moon, Purnima, and the new Moon, Amavasya. Plenty of people keep a Purnima vrat or remember their ancestors on Amavasya. Fewer know why the purnima vrat date sometimes falls on the evening before the Purnima shown in the calendar.

That is not a mistake on anybody's part. Two different rules are at work, and once you can see both, the calendar makes sense.

A tithi is not a day

Both days are tithis, lunar days. A tithi is measured by the distance between the Moon and the Sun, not by the clock, so it begins and ends at any hour. One tithi might start just after lunch and finish the next afternoon. Another might start and end between two sunrises.

So the calendar has to choose. For a given observance, which moment of the day decides whether the tithi "belongs" to it? The tradition answers that differently for different observances, and that is the whole story behind two dates.

Purnima: what it is for

The full Moon is traditionally a bright, giving day. It is kept for:

• a ritual bath, ideally in a river, and daan, giving to others
• named festivals that fall on particular full Moons: Guru Purnima, Buddha Purnima, Kartik Purnima and others
• the Purnima vrat, a fast traditionally broken after seeing the Moon

Many families also hold a Satyanarayan katha at Purnima, though households differ on which of the two days they choose. The bath, the giving and the named festivals belong to the day. The fast belongs to the night, and that is where the split comes from.

How the Purnima vrat date is decided

This site applies two separate rules:

• Purnima day is the day on which Purnima prevails at sunrise. This is the day for the bath, daan, and Guru, Buddha or Kartik Purnima.
• Purnima vrat is the day on which Purnima prevails at moonrise. Because the fast is completed by sighting the Moon, the Moon's rising is the moment that counts.

Near full Moon, the Moon rises around sunset. If Purnima begins during the afternoon, it is already running when the Moon comes up that evening, but the next morning's sunrise also falls inside it. The vrat goes to the first evening; the Purnima day goes to the next morning.

The Guru Purnima 2026 example

This year showed it clearly. For Delhi, our engine placed:

• Guru Purnima, the Purnima day: 29 July 2026, since Purnima prevailed at sunrise that morning
• the Purnima vrat: 28 July 2026, since Purnima was already running at moonrise the evening before

Two families in the same city could both have been right. One fasted on the 28th and broke the fast at moonrise; the other offered thanks to their guru on the morning of the 29th. They were answering different questions.

It does not happen every month. Often both rules land on the same day. When they do split, it is the vrat that can fall on the evening before.

Amavasya: what it is for

The new Moon is quieter in tone. Traditionally it is the day for the ancestors: tarpan, offering water and sesame, and shraddha, remembering the dead by name. It is also kept for bathing, giving and quiet observance. Many people avoid starting new ventures on it, though that is a custom rather than a rule written into the calendar.

This site keeps Amavasya on the day it prevails at sunrise, the same rule as the Purnima day. The rituals belong to the day.

Somvati, Shani, Mauni and Mahalaya Amavasya

Some new Moons carry a name of their own. The names do not change the rule; they say why that particular Amavasya is traditionally given more weight.

Somvati Amavasya is an Amavasya that falls on a Monday. Soma is the Moon, and Monday is his day, so the combination is considered especially fitting for bathing and, by tradition, for prayers for a husband's long life. In many places married women walk round a peepal tree on this day.

Shani Amavasya, or Shanishchari Amavasya, is an Amavasya on a Saturday. It is traditionally linked with Shani, and people going through Sade Sati or a dhaiya often choose it for Shani worship and giving.

Mauni Amavasya is the Amavasya of Magha (Pausha in the amanta reckoning, where the month ends at the new Moon). Its name comes from mauna, silence: the custom is to keep silent for the day and bathe, and it is one of the most important bathing days at Prayag.

Mahalaya Amavasya, also called Sarva Pitru Amavasya, closes Pitru Paksha, the fortnight given to the ancestors. Anyone who does not know the tithi on which a relative died can offer shraddha on this day for all of them together. In Bengal it also marks the start of the Durga Puja season.

Whether a Monday or a Saturday Amavasya falls in a given year depends on the weekday the sunrise rule lands on, which is one more reason to check rather than assume.

Why your city changes the date

Sunrise and moonrise are local. The tithi changes at the same instant everywhere, but that instant falls at a different hour of each city's day, so two places can disagree about which date a tithi belongs to.

This matters a great deal for anyone abroad. Karwa Chauth, another fast broken at moonrise, shows it this year: our engine puts it on 29 October in Delhi, with moonrise at 8:11 PM, but on 28 October in New York, with moonrise at 7:29 PM. Rama Ekadashi follows the same pattern: 5 November in Delhi, 4 November in New York and London. A Purnima or Amavasya worked out for Delhi can likewise be a day off for you. A date copied from a family WhatsApp group in India is the right answer for India, not necessarily for where you live.

That is why our panchang is calculated for the place you give it, not for one city in India. If you are in the UK, the London panchang shows the day's tithi with London's own sunrise. If tithis and the other limbs of the day are new to you, the five limbs of the panchang is a good starting point.

Find the dates for your city

Our Purnima dates for 2027 give the Purnima day for each month, worked out for your city, and show the vrat on the evening before whenever the two rules split, along with moonrise. The Amavasya dates for 2027 apply the sunrise rule in the same way, and name the Somvati, Shani, Mauni and Mahalaya Amavasyas where they fall.

Every date is calculated from an ephemeris for your location rather than copied from a printed table. The calendar is only telling you where the Moon is; which customs you keep is yours to decide.