Tithi

A tithi is the time the Moon takes to gain twelve degrees on the Sun. There are thirty in a lunar month, so a tithi is usually a little shorter than a calendar day and does not begin at midnight. Its length varies by several hours from tithi to tithi.

The thirty are split into two halves of fifteen. In the bright half, Shukla paksha, the Moon is waxing and the tithis run from Pratipada to Purnima, the full Moon. In the dark half, Krishna paksha, they run from Pratipada to Amavasya, the new Moon.

The tithi that is running at sunrise is the one commonly used to name the day for festivals and observances, which is why a tithi can appear to be skipped or repeated in a calendar.