When someone in Mumbai observes a fast on Ekadashi and a cousin in Vancouver does the same, they may not be fasting on the same civil date. The reason lies in the tithi, the lunar day that forms the backbone of the Hindu calendar. Unlike the midnight-to-midnight day used for appointments and train schedules, a tithi is an astronomical event tied to the dance between sun and moon.
What a tithi measures
A tithi is the time required for the moon to gain exactly twelve degrees of longitude over the sun. Because a full lunar cycle covers three hundred sixty degrees, there are thirty tithis in each lunar month. The word itself comes from the Sanskrit root meaning "a lunar day," but it is not a day in the sense of Earth's rotation. It is a slice of the moon's monthly journey.
The moon moves through the sky at varying speeds because its orbit is elliptical, not circular. When the moon is closer to Earth, it travels faster; when farther, slower. This means a tithi can last anywhere from about nineteen hours to nearly twenty-six hours. Most are close to twenty-four, but the variation matters.
Why the tithi begins at sunrise
Vedic timekeeping anchors the tithi to sunrise because that is when the visible world wakes and religious duties traditionally begin. Midnight is an abstract point, invisible without a clock. Sunrise is an event anyone can witness.
When you consult a panchang, the daily almanac used across India and the diaspora, it lists which tithi is current at sunrise in your location. If a tithi ends before the next sunrise, that tithi is said to be "lost" for ritual purposes in some traditions. If a tithi spans two sunrises, it may be counted twice, though practice varies by region and sampradaya.
This is why a vrat or festival can fall on different Gregorian dates depending on where you live. The panchang for Toronto will differ slightly from the one for Delhi, not because the moon behaves differently, but because sunrise does.
The fifteen tithis of each fortnight
Each lunar month is divided into two fortnights: the bright half, when the moon waxes, and the dark half, when it wanes. Each fortnight contains fifteen tithis.
| Tithi number | Bright fortnight (Shukla Paksha) | Dark fortnight (Krishna Paksha) |
|---|---|---|
| 1 | Pratipada | Pratipada |
| 2 | Dwitiya | Dwitiya |
| 3 | Tritiya | Tritiya |
| 4 | Chaturthi | Chaturthi |
| 5 | Panchami | Panchami |
| 6 | Shashthi | Shashthi |
| 7 | Saptami | Saptami |
| 8 | Ashtami | Ashtami |
| 9 | Navami | Navami |
| 10 | Dashami | Dashami |
| 11 | Ekadashi | Ekadashi |
| 12 | Dwadashi | Dwadashi |
| 13 | Trayodashi | Trayodashi |
| 14 | Chaturdashi | Chaturdashi |
| 15 | Purnima (full moon) | Amavasya (new moon) |
Certain tithis carry special significance. Ekadashi, the eleventh tithi, is observed with fasting in many traditions. Chaturthi honors Ganesha. Ashtami and Navami are central to Navaratri. The vrat calendar on this site lists these observances so families in North America can plan around work and school.
How this affects ritual timing
When a priest says a ceremony should be performed "on Akshaya Tritiya," he means during the period when that tithi is current, not simply on May third or whenever the Gregorian date falls. If the tithi begins at dawn and ends by noon, the auspicious window is those few hours, not the entire civil day.
For someone living in an apartment in New Jersey, far from a temple with a resident pandit, this can be confusing. A panchang app may show the tithi changing at seven in the morning local time, but a relative in India might be observing the same event a day earlier or later because their sunrise is different. Neither is wrong; both are following the same lunar rhythm in different time zones.
Regional variations you will encounter
Not all panchang publishers agree on every detail. Some follow the amanta system, ending the month on the new moon. Others use purnimanta, ending it on the full moon. Some calculate planetary positions using the drik method, which relies on direct observation and modern ephemerides; others use vakya, an older formula-based approach. These choices can shift a tithi's start time by a few hours.
If your family's tradition comes from Tamil Nadu, the panchang you grew up with may differ slightly from one published in Gujarat or Bengal. This is not error; it is the diversity of living practice. When in doubt, follow the custom of your lineage or consult the panchang section of this site, which notes these differences plainly.
Living with lunar time in a solar world
For families keeping a foot in both calendars, the trick is not to force the tithi into a box it was never meant to fit. Mark the Gregorian date on your phone, but understand that the observance itself is timed to the moon. If you wake to find the tithi has already ended, tradition often allows the vrat to be kept the previous day or the practice adjusted.
The tithi is not a rule handed down to make life difficult. It is a way of noticing that time has texture, that the sky is not a backdrop but a participant in the rhythm of devotion.