The Lunar Day

A tithi is a lunar day in the Hindu calendar: the time it takes the angular distance between the Sun and Moon to increase by twelve degrees. Because the Moon's motion is uneven, tithis vary in length and do not align neatly with solar days, a tithi may be a little shorter or longer than a day, and one can occasionally be skipped or repeated.

There are thirty tithis in a lunar month, fifteen in the waxing fortnight (shukla paksha) and fifteen in the waning (krishna paksha), each with a name and a character, and many Hindu festivals and observances are fixed to a particular tithi rather than to a solar date.