once in a blue moon

If something happens once in a blue moon is happens only very rarely.

once in a blue moon意思是非常罕見的事。

An actual blue moon-that is, the appearance of a second full moon in the same calendar month-occurs every 32 months or so. 

真正的藍(lán)月亮——是指，一個月內(nèi)的第二次滿月——大約每32個月出現(xiàn)一次。

He’s the kind of player that comes along once in a blue moon.

他是那種百年不遇的天才球員。

This is a chance once in a blue moon.

這是個千載難逢的好機(jī)會。