in a blue moon

基本解释

From this perspective, unused functions should not be dropped from software merely because they are used once in a blue moon.
從這個觀點來說，不習慣使用的功能不應該僅僅從軟件中去除因爲他們往往給使用他們的用戶一種千載難逢的感覺。
To keep the game interesting, most of it is paid out in dribs and drabs: a machine that returned $95,000 of every $100,000 in one huge jackpot once in a blue moon would be too disheartening to play.
爲了讓賭戯保持吸引力，機器廻餽的獎金都是零零碎碎的：如果在一廻千載難逢的機遇下一次性返還獎池內10萬美元的95%則太過令人沮喪，玩興全無。
In English we say “once in a blue moon”.
“千載難逢”用英語我們說“once in a blue moon” 。
edu.sina.com.cn

Occasionally it will be hundreds of millions and once in a blue moon billions.
FORBES: How To Regulate Robots With Fat Fingers
And then, every once in a blue moon, something comes along that is in a category all its own.
FORBES: Live Like a Tycoon in This $78 Million Dollar Mansion
Once in a blue moon, he tells me, he saw an iPhone.
FORBES: Apple Rolls Out Installment Payment Plan in China