"Once in a blue moon..." It's a cliche that we hear often. It's meaning is that something only happens once in a while.
A blue moon may not live up to the cliche's name, though. A blue moon happens every 2.7 years.
The cycle of lunar phases takes 29.53 days. That's less than a month (unless you're talking about February).
One year has 365.26 days.
A little math shows that more than 12 lunar cycles happen each year, but there are only 12 months in each year.
365.26 days per year / 29.53 days per lunar cycle = 12.37 lunar cycles per year
The extra .37 gets added year after year.
It works out, then, that an extra full moon (the 13th in a year) occurs every two or three years.
This extra full moon, then, becomes the "blue moon" for the month that it occurs.
Of course, the moon doesn't actually turn blue when this happens. The term "blue moon" probably stems from one folklore. Perhaps it originated from a Latin word meaning "early" when a full moon occurred too early for lent. No one seems to know the answer to the question of a "blue moon's" origin.
7831 N. Business Park Drive
Tucson, AZ 85743