This is a chance once in a blue moon.

这可是个难得的机会。

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　　固定搭配once in a blue moon，意思是“偶尔、罕见(every now and then, rarely)”。传统习惯上，blue moon相当于中国农历里的“闰月”。现代英语中，blue moon的意思有些变化，相当于“很长一段时间，”（A relatively long period of time），因此，onec in a blue moon字面的意思就是，很长时间才发生一次，就是“偶尔”的意思了。
　　例句：
　　Barry only visits his parents once in a blue moon.
　　巴里难得去看望他父母一次。
　　I haven't seen you in a blue moon. 我很久没见到你了。
　　It snows here once in a blue moon.
　　这里几乎不下雪。（这里很长时间才下一次雪。）

This is a chance once in a blue moon.
This is a chance once in a blue moon.

Barry only visits his parents once in a blue moon.
　　巴里难得去看望他父母一次。
　　I haven't seen you in a blue moon. 我很久没见到你了。
　　It snow here once in a blue moon.
　　这里几乎不下雪。（这里很长时间才下一次雪。）
Barry only visits his parents once in a blue moon.
I haven't seen you in a blue moon.
It snow here once in a blue moon.

I haven't seen you in a blue moon.

This is a chance once in a blue moon.

this a chance once in a blue moon.
this a chance once in a blue moon.
this a chance once in a blue moon.
this a chance once in a blue moon.

Barry only visits his parents once in a blue moon.
Barry only visits his parents once in a blue moon.

this is a chance once in a blue moon.

this is a great chance once in a blue moon.
he visit his parents once in a blue moon.

this is a chance once in a blue moon.
this is a chance once in a blue moon.
this is a chance once in a blue moon.
this is a chance once in a blue moon.