def>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 moonvery 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 ]
[千载难逢 / 千載難逢, qiān zǎi nán féng, ㄑㄧㄢ ㄗㄞˇ ㄋㄢˊ ㄈㄥˊ]lit. hard to meet in a thousand years (成语 saw); extremely rare opportunity; once in a blue moon#32163[Add to Longdo]