01 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,…
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1.
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.
Phrases & compounds
once in a blue moon —
very rarely; -- from the observation that the moon rarely has a bluish tint.