A lunar day is one of
the thirty days that make up a lunar month, running from full moon to full
moon.
Each lunar day is somewhat shorter than a solar day since
these thirty lunar days occur in around twenty-eight sun days.
Even in modern times, most Hindu religious observances are
dictated by the lunar calendar, making the calculation of these lunar days
crucial.
The majority of people use a panchang, an almanac that lists
all of the lunar days, to keep track of these celebrations.