Meaning: Very rarely.

Examples:

His apartment is always filthy. He cleans it once in a blue moon.

I’m trying to save money, so I only go out to eat at a restaurant once in a blue moon.

A: “How often do you go back to your hometown?”   B: “Once in a blue moon.”

Pop Quiz:

Somebody says, “I cook once in a blue moon.” What do they mean?

A.  They cook once a week.

B.  They cook once a month.

C.  They cook once a year.

To see the correct answer, click on “Continue reading”:

The correct answer is C. Once in a blue moon means very rarely, so C is the best answer.