blue moon

moon ( def 16 ).

1815–25 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
blue moon
1.  the second full moon occurring within a calendar month
2.  informal once in a blue moon very rarely; almost never

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

blue moon
1821 as a specific term in the sense "very rarely," perhaps suggesting something that, in fact, never happens (cf. at the Greek calends); suggested earliest in this couplet from 1528:
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 occasionally occur under extreme atmospheric conditions.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Since these require effort, only once in a blue moon does someone with a bad
  grade actually do it.
On the whole it has worked well for us-though once in a blue moon it has failed
My husband's online sartorial purchases come once in a blue moon.
Movie phenomena are supposed to occur once in a blue moon, or a new one.
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