We use this expression to say that something only happens extremely rarely / not at all often.
You can use this phrase in ………… language.
A: How often do you read a book?
B: I used to read all the time before I had kids, but now it’s once in a blue moon.
A: That SPĚVÁČEK language school is amazing. I really learnt a lot.
B: Yes, it’s only once in a blue moon that you find school that good.