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。