Why are lunar months a problem for lunar calendars?
A lunar calendar is based on lunar months (the cycle of the moon’s phases takes 28 days). Since this cycle is fairly constant, the months all have the same length. The problem arises because no whole number of lunar months corresponds to one year; the closest is 13 months, equal to 364 days. This leaves an overhang of one and a quarter days each year which must be accounted for in a similar manner to leap years.
