blue moon (&unr_;) 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 the old phrase blue moon has become widely accepted.
[PJC]