You only get a glance like this once in a blue moon!

题目

You only get a glance like this once in a blue moon!

答案

固定搭配once in a blue moon,意思是“偶尔、罕见(every now and then,rarely)”.
整句话的中文意思应该是：你只是一瞥了这千载难逢的奇观.