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)