GNU Collaborative International Dictionary of English

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1.       
   blue moon  The second full moon occurring in the same month; -- derived from the expression once in a blue moon; as, we had a blue moon on January 31, 1999, and another in March.
   PJC
   once in a blue moon very rarely; -- from the observation that the moon rarely has a bluish tint. In 1980, the phrase blue moon was applied to the second full moon of any month, because this phenomenon is unusual. This new meaning for blue moon, a long time, used primarily in the phrase once in a blue moon, has become widely accepted.
   PJC