Tithi — the lunar day
A tithi is the time the Moon takes to gain 12 degrees of longitude on the Sun. Thirty tithis make one lunar month, split into the waxing fortnight (shukla paksha) and the waning fortnight (krishna paksha).
Because the Moon travels at an uneven speed, a tithi is not a fixed length. It can run from roughly 19 to 26 hours, which is why a tithi rarely lines up with a calendar day and may occasionally be skipped or repeated.