blue moon
noun [ C ] environment specialized (also Blue Moon) uk/ˌbluː ˈmuːn/ us/ˌbluː ˈmuːn/
（每两年半出现一次的）蓝月亮，一个月中的第二个满月
the second full moon (= the time when the moon is shaped like a complete disc) in a month, that appears around every two and a half years
How often does a Blue Moon occur, and when will there be another? 蓝月亮多久会出现一次，下次什么时候？
A blue moon doesn't look blue. 蓝月亮看起来并不是蓝色的。
- More examples
- The second full moon in any month is called a blue moon.
- Depending on your time zone, the Blue Moon of July 2004 occurred in either July or a month later
- October did not feature a blue moon, contrary to widespread reports.
Idiom
once in a blue moonidiom 少有地
not very often
My sister lives in Alaska, so I only see her once in a blue moon. 我姐姐住在阿拉斯加，所以我很少见到她。