Once in a Blue Moon..... 
I like this expression because it's true that "Blue Moon's" don't happen all that often. Maybe once a year? One time, in a recent calendar year, there were two occasions of an "Blue Moon."
So, if one uses this particular expression and you know that the person rarely does x, y or z and they use this particular way of describing their situation, then I think it's reasonable to believe that it was truly an 'Once in a Blue Moon' experience.