Tithi (lunar day)
A tithi is a lunar day: the time the Moon takes to move 12° further ahead of the Sun. There are 30 tithis in a lunar month, 15 in each paksha, and the tithi in force at sunrise names the day.
Also known as: Thithi
As the Moon pulls ahead of the Sun, the angle between them grows from 0° at new moon to 360° at the next. Each 12° step is one tithi. Because the Moon’s speed varies, a tithi can last from about 20 to almost 27 hours, so it rarely lines up with the calendar day.
The first 15 tithis, Pratipada to Purnima, form the bright half (Shukla paksha); the next 15, Pratipada to Amavasya, form the dark half (Krishna paksha). Panchangs name each day by the udaya tithi, the one in force at sunrise. Occasionally a tithi begins and ends between two sunrises and is skipped (kshaya), or spans two sunrises and repeats (vriddhi).
Tithis set most fasts and festivals, from Ekadashi to Diwali, and shraddha is performed on the tithi of a loved one’s passing. Some observances are judged at another time of day, such as Pradosh at sunset, Shivaratri at midnight and Sankashti at moonrise, so a vrat can fall on a day the panchang labels with a different tithi.