not happen (/seen) once in a hundred years (/a century) ❍ 不是要开会吗？ ～的事，…… (《老舍剧作选·龙须沟》 62) Have you forgotten the meetings today! This is something that doesn’t happen once in a hundred years.
百年不遇bǎi nián bù yù
形容很难遇到。very rare occasions,rarely meet in a century