Where we teach you all about idioms in American English idiom…
“Once in a blue moon.”
It doesn’t mean that the moon is blue. So let’s listen to this American English conversathion to find out how it is uaed.
A : Where did you say your brother lives?
B : He lives all the way in Minnesota, so I never get to see him. I only get to see him once in a blue moon.
Once in a blue moon is a common way of saying that something does not happen often. But what is a blue moon? We typically see a full moon once a month. And seeing two full moons in the same month is rare. The second full moon in a particular month is called a blue moon. Once in a blue moon is an informal phrase.