From Longman Dictionary of Contemporary Englishonce in a blue moononce in a blue mooninformalRARELY very rarely It only happens like this once in a blue moon. → once
Examples from the Corpus
once in a blue moon• I used to spend a lot of time in London, but now I only go there once in a blue moon.• We go out to eat once in a blue moon.• And Eleanor was damn lucky to have him as an escort once in a blue moon.• So now he just comes round once in a blue moon.• Once in a blue moon Eric will offer to help with the dishes, but usually he doesn't do any housework at all.• That happens only once in a blue moon, when the weather is cold enough and thus the ice thick enough. 