千载难逢--------It only happens once in a blue moon

例句：
A: Let's go to the megastar's singing performance, shall we?
我们去听那个天王巨星的演唱会，好吗？
B: Why not? It only happens once in a blue moon.
好啊！那可是个千载难逢的机会。

同义词：

It chances in an hour that happens not in seven years