A tithi is a lunar day. It is the time the Moon takes to move 12° further away from the Sun in the sky.
30 tithis in a lunar month
The Moon gains 360° on the Sun in one lunar month, so there are 360 ÷ 12 = 30 tithis. The first 15 (Pratipada to Purnima) form Shukla Paksha; the next 15 (Pratipada to Amavasya) form Krishna Paksha.
Tithis don't follow the clock
The Moon's speed changes along its orbit, so a tithi can last anywhere from about 19 to 26 hours, and it can start at any time of day. Sometimes a tithi begins and ends between two sunrises and is "skipped" (kshaya tithi); sometimes one covers two sunrises (vriddhi tithi).
How a festival day is chosen
Each festival has a rule for which moment of the day the tithi must cover. For example:
- Karva Chauth, Sankashti Chaturthi: the Chaturthi should prevail at moonrise.
- Diwali: Amavasya should prevail during Pradosh Kaal, just after sunset.
- Ganesh Chaturthi: Chaturthi at midday.
- Most Purnima festivals: the tithi at sunrise (udaya tithi).
Because tithi times are the same everywhere while sunrise and moonrise change by place, two calendars (or two regions) can occasionally put the same festival on different days.
Amanta and Purnimanta months
North India usually ends a month on Purnima (Purnimanta), while Maharashtra, Gujarat and South India end it on Amavasya (Amanta). The festival is the same; only the month name of the Krishna Paksha differs. Karva Chauth is "Kartik Krishna Chaturthi" in the north and "Ashvin Krishna Chaturthi" in Amanta calendars.