This is a chance once in a blue moon.
这是个千载难逢的机会。
blue moon 在现代天文学意义上指：“一个月中的第二次满月”，由于月运周期是29.5天，而根据日历，每个月大多数都有至少30天，因此，大约每32个月才会出现一次一个月两次圆月的现象。所以这个习语用以形容这种罕见的天文现象，后来引申为：“极为罕见的事情，千载难逢的机会”。因此，当美国人说"This is a chance once in a blue moon."时，他/她要表达的意思就是:"It is a golden opportunity."、"It is an once-in-a-lifetime opportunity."。
情景对白：
Terry: Frankly speaking, the boss always thinks highly of you. Why do you want to leave?
泰瑞：坦白地说，老板一直都很器重你。你为什么要走呢?
Benjamin: A friend of mine introduced me to his company which I have been dreaming to enter.This is a chance once in a blue moon.
本杰明：一个朋友介绍我去他们公司，那可是我做梦都想去的公司啊。这是个千载难逢的机会。
搭配句积累：
①I've got an offer of that company which is number one in this industry.
我收到了那家公司的录用函，它在行业内可是排名第一的哦。
②That company seldom holds outside recruit for senior managers.
那家公司很少从外部招聘资深经理。
③My boss allows me to resign before time.
老板允许我提前辞职了。
④My research has gained support from the government. So I'd like to quit.
我的研究已取得政府支持，所以我想辞职。
单词：
1. once-in-a-lifetime 千载难逢的
You can't pass up this job. This kind of opportunity comes only once in a lifetime.
你不可错过这个工作。这种机会一辈子才有一次。
It's definitely a once-in-a-lifetime experience.
这绝对是一次毕生难忘的体验。
2. before time 提前
Against all the odds, I finished my work well before time that I wanted to leave.
尽管非常困难，在我打算要离开之前，我早早完成了工作.
eg. She went there before time , taking the papers with her.
她带着文件提早到那里去。
