Tithi

A lunar day, that is to say, one of the thirty
days occurring during a single lunar
month, from full moon to full moon.
Because these thirty lunar days take
place in about twenty-eight solar days,
each lunar day is thus slightly shorter
than a solar day. Even in contemporary
times, most Hindu religious observances are determined by the lunar calendar, which makes the determination
of these lunar days an important matter.
Most people keep track of these holidays
with a panchang, an almanac that gives
all the lunar days.