ph.

1. Such a chance comes once in a blue moon.
这样的机会极其难得。

1. 出蓝月亮的时候(稀罕,少见)：on the tip of my tongue 话到舌尖,呼之欲出 | once in a blue moon 出蓝月亮的时候(稀罕,少见) | one foot in the grave 一脚已经入了坟(入土三尺)

2. 千载难逢：be blue in the face(由于愤怒或过分费力) 脸色突变 | once in a blue moon 千载难逢 | sing the blues 垂头丧气

3. 机会难得;绝无仅有：on top of the world 精神饱满 | once in a blue moon 机会难得;绝无仅有 | once in a while 偶尔

4. 稀罕：As soon as possible! 越快越好! | ? once in a blue moon稀罕; | ?two left feet笨手笨脚;