One lunar cycle) is 29.53 days. There are about 365.25 days in a solar
year. Therefore, about 12.37 cycles (365.25 days divided by 29.53 days)
occur in a solar year. In the Gregorian calendar, there are 12 months
(derived from the word moon) in a year, and normally there is one full
moon each month. Each calendar year contains roughly 11 days more than the
number of days in 12 lunar cycles. The extra days accumulate, so every two
or three years there is an extra full moon. The extra moon necessarily
falls in one of the four seasons, giving that season four full moons
instead of the usual three, and, hence, a blue moon.