Blue–moon

moon (def 16).
Contemporary Examples

The Beer Summit Mark Katz July 27, 2009
Beck’s Musical Time Machine: This Wasn’t a Concert. It was a Spectacular Party. Andrew Romano April 17, 2014
Lights, Camera, Cocktails Brody Brown June 30, 2011
Obama’s Power Ties Vanessa Friedman November 30, 2009
Lights, Camera, Cocktails Brody Brown June 30, 2011

Historical Examples

Northern Spain Edgar T. A. Wigram
The Cat in Grandfather’s House Carl Henry Grabo
Motor Boat Boys’ River Chase Louis Arundel
The Mystery of Carlitos Helen Randolph
The Motor Boys Afloat Clarence Young

noun
the second full moon occurring within a calendar month
(informal) once in a blue moon, very rarely; almost never
n.

Yf they say the mone is blewe,
We must beleve that it is true.

Though this might refer to calendrical calculations by the Church, so that the general sense of the term and the specific one (commonly misinterpreted as “second full moon in a calendar month,” but actually a quarterly calculation) are difficult to disentangle. In either case, the sense of blue is obscure. Literal blue moons do sometimes occur under extreme atmospheric conditions.