once in a blue moon
It means "very rarely", and comes from the English meaning: a month in which there are two full moons (they occur, but not often). Is there a similar expression in Chinese? hanzi + pinyin would be greatly appreciated :)

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太阳从西边出来了
哈哈哈

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千载难逢 literally means it happens only once in a thousand years
pin yin :   qiān　zǎi　nán　féng

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yes，千载难逢 is the accurate and exact equvalence of the idiom once in a blue moon, which means it is a really hard-won occasion or chance,etc~~~have a nice day