Tithi
A lunar day defined by each 12-degree step of the Moon–Sun angular separation.
Definition
There are 30 Tithis in a synodic lunar cycle: 15 in Shukla Paksha and 15 in Krishna Paksha. A Tithi can begin or end at any clock time, so it does not map one-to-one to a Gregorian date.
Example
If the Moon–Sun separation crosses the next 12-degree boundary at 8:25 AM, the Tithi changes at that time even though the civil date stays the same.
Festival and Vrat rules may ask which Tithi prevails at sunrise or during a specific time window; those observance rules are separate from the raw Tithi calculation.