a blue moon

基本解释

If you only send faxes once in a blue moon, do it from Staples or a copy shop.
如果您只发送传真一次在一蓝色的月亮，也从史泰博或复印店了。
www.internet.org.cn
You should be grateful for the extra job offer. Things like that come along only once in a blue moon.
你应该对额外的工作机会抱有感激之心：这样的事可是极其难得。
He’s the kind of player that comes along once in a blue moon, a play maker whose vision, passing and inventiveness makes a huge impact on any game he plays in.
他是那种百年不遇的天才球员。作为球队的核心，他的视野，激情以及创造性都会对比赛产生巨大的影响。

Occasionally it will be hundreds of millions and once in a blue moon billions.
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And then, every once in a blue moon, something comes along that is in a category all its own.
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Once in a blue moon, he tells me, he saw an iPhone.
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