Once in a blue moon

How often do you see a blue moon?

Not very often?

Then something that happens very infrequently happens once in a blue moon.

E.G. 'Do you go to the movies very often?'

'Hardly at all; once in a blue moon.'

Of course, blue moon isn't just an idiom but a classic rock and roll ballad - like this:

Once in a blue moon

Have you ever seen a blue moon?

I haven't; I guess it's something that happens very rarely.

Like, once in a blue moon.

It's quite an old idiom as indicated by this very old song from 30's and 40's heart-throb, Bing Crosby, who you might know for his most famous song, White Christmas.

His style of singing is called crooning and that makes him a crooner. Of course, you don't hear this sort of music very often now; just once in a blue moon.

(Thanks to Dorota for giving me the idea for today's idiom)