If something happens once in a blue moon is happens only very rarely.
once in a blue moon意思是非常罕见的事。
An actual blue moon-that is, the appearance of a second full moon in the same calendar month-occurs every 32 months or so.
真正的蓝月亮——是指,一个月内的第二次满月——大约每32个月出现一次。
He’s the kind of player that comes along once in a blue moon.
他是那种百年不遇的天才球员。
This is a chance once in a blue moon.
这是个千载难逢的好机会。