'once in a blue moon' definitions:
Definition of 'once in a blue moon'
From: GCIDE
- blue moon \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.
- Note: 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]