Once in a Blue moon

“Once in a blue moon” is another popular expression in English that has something to do with color.

This phrase means “a rare event” and, although it originated as a reference to the moon, it is used in other contexts as well.

According to experts, a blue moon used to refer to the third full moon out of four in every season. However, the more accepted theory is that it refers to the second full moon that occurs in a given calendar month.

Since there are only 41 months in every century that contain two full moons, a “blue moon” only occurs once every two and a half years. However, the phrase “once in a blue moon” does not necessarily mean once every two and a half years.

It is merely used to describe some rare event no matter if the time between occurrences is two years orlonger. Here is an example of how the expression is used:

He only shows up once in a blue moon to collect the rent.

Here is another example of how this phrase is used:

http://robinlee.typepad.com/new_in_christian_fiction/2010/03/once-in-a-blue-moon-by-leanna-ellis.html

(see last line in 1^st paragraph)