Introduction
The panchang has five limbs: tithi, vaar (weekday), nakshatra, yoga and karana. The tithi is the one families use most. When someone at home asks "which tithi is it tomorrow?", they are really asking how far the Moon has moved away from the Sun.
A tithi is not a clock day. It comes from the Moon's motion, so one tithi may last about 20 hours and another about 26. That is why a panchang prints an end time next to every tithi.
How a tithi is formed
At amavasya (new moon) the Sun and Moon are close together. The Moon then moves faster and pulls ahead. While the Moon–Sun gap is 0° to 12°, the first tithi (Pratipada) runs; from 12° to 24°, the second (Dwitiya); and so on.
360° divided by 12° gives 30 tithis. B. V. Raman's A Manual of Hindu Astrology gives this method of calculation. ePandit's engine works the tithi out from the Sun and Moon positions at sunrise in your own city.
Shukla paksha and Krishna paksha
The first 15 tithis make the Shukla paksha, the bright half: the Moon appears to grow, and the 15th tithi is Purnima, the full moon. The next 15 make the Krishna paksha, the dark half: the Moon appears to shrink, and the 30th tithi is Amavasya.
The names repeat in each half: Pratipada, Dwitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi, Chaturdashi. So "Shukla Ekadashi" and "Krishna Ekadashi" are different days. Neither half is better than the other; each has its own festivals.
The five tithi groups
The Muhūrta-cintāmaṇi sorts the tithis into five groups, which repeat three times in each paksha:
| Group | Tithis |
|---|---|
| Nandā | 1, 6, 11 |
| Bhadrā | 2, 7, 12 |
| Jayā | 3, 8, 13 |
| Riktā | 4, 9, 14 |
| Pūrṇā | 5, 10, 15 / 30 |
The names carry their meaning: Nandā means joyful, Bhadrā means kindly, Jayā means victory, Riktā means "empty", and Pūrṇā means full. Tradition usually does not pick the Riktā tithis to begin something new. That does not mean anything will go wrong on those days: Chaturthi is Ganesha's day and Navami is the Goddess's, both kept with great love.
(The "Bhadra" that people avoid is a karana, which is a different thing; see the karana page.)
Why the sunrise tithi counts
For most vrats and festivals, the tithi that counts is the one running at sunrise, even if it ends by noon. Decision texts such as the Dharmasindhu set out rules of this kind in detail, and some festivals have their own rule. Janmashtami, for example, looks at the tithi at midnight.
So when you read a panchang, check two things: which tithi is running at sunrise, and until what time.
Skipped (kshaya) and repeated (vriddhi) tithis
Sometimes a short tithi begins after one sunrise and ends before the next. It never touches a sunrise, so it seems to vanish from the calendar. This is a kshaya tithi.
Sometimes the opposite happens: a long tithi touches two sunrises, so the same tithi is written on two days. This is a vriddhi tithi.
Both follow simply from the Moon's changing speed. It is also why two panchangs can differ by a day: a different city has a different sunrise.
Ekadashi: Smarta and Vaishnava rules
Many people keep the Ekadashi fast, and calendars sometimes list a "Smarta Ekadashi" and a "Vaishnava Ekadashi" on different days. There is nothing to worry about.
- The Smarta tradition broadly goes by the tithi at sunrise.
- The Vaishnava tradition also looks at arunodaya, roughly an hour and a half before sunrise. If Dashami is still running then, they keep the fast on the next day.
Both rules are old and both are respected. Follow the sampradaya or family tradition your household belongs to.