Yeah, I like that better, actually, so I'll put the literal translation in the Remarks section instead!
Thanks, lilian!
Thanks for your version too, Angelus, but I don't think the expression "blue moon" (taken from the standard expression "once in a blue moon," meaning extremely rarely, i.e. an expression about frequency, not about the length of time as such) could be used in this way. Usually we can't take idioms apart and use just pieces of them (I try to do that in Chinese too, and often look foolish! ): they are usually kinds of all-or-nothing expressions that are not "dissectable"!