How often do you see the blue moon? I have never seen it myself; and it rarely happens. So when you say, “once in a blue moon,” it means “rarely.” (어쩌다가 한번씩 또는 가뭄에 콩나듯이)
a. How often do you hit the gym? (Do you remember that “hit places” means that you visit there? I covered “hit” usage in the previous post. 얼마나 자주 운동하러 짐에 가세요?
b. Once in a blue moon. (아주 가끔씩. 가뭄에 콩나듯이)