tithis

Tithis are lunar days in the Hindu lunisolar calendar. A tithi is the time duration in which the longitudinal angle between the Sun and the Moon differs by 12 degrees. Since the Moon moves approximately 12 degrees per day relative to the Sun, a tithi is typically about 24 hours, but can be shorter or longer. There are thirty tithis in a lunar month, which is divided into two halves: Shukla Paksha (the waxing phase of the moon) and Krishna Paksha (the waning phase of the moon). Each paksha has fifteen tithis.

The names of the tithis are numbered from one to fourteen, followed by the Amavasya (new moon) and Purnima (full moon). For example, Pratipada is the first tithi, followed by Dwitiya, Tritiya, and so on. The day of the week on which a particular tithi falls is less significant than the tithi itself in Hindu astrological calculations and for determining auspicious timings for religious observances, festivals, and ceremonies. The duration of a tithi can vary due to the varying speeds of the Sun and Moon. When a tithi is shorter than 24 hours, it is called a Kshaya tithi. Conversely, when a tithi lasts longer than 24 hours and overlaps with the next solar day, it is called a Vriddhi tithi.