4 KudoZ points awarded for this answer. Comment: "Zahvaljuem Miro kao i svim ostalim kolegama! Zaista sam morao biti oprezan sa ovim terminom obzirom da nema mnogo veze sa cjelokupnim tekstom koji prevodim. 'Once in a blue moon' means once every two-and-a-half years Simon. Even it sounds like it is a colour but it isnt. But nly after volcanic eruptions or unusual weather conditions. :)"