Once in a blue moon

Once in a blue moon

Meaning:

Rarely, almost never

If you say something happens once in a blue moon, you mean it happens very rarely.

A blue moon is the second of two full moons in the same month, which happens rarely.

Example:

He has a very nice car, but he drives once in a blue moon.

A: Does your husband cook for you?
B: Oh, yes, once in a blue moon!