once in a lifetime (/in a thousand years); a rare opportunity ❍ 现在慈、光死了，真是～的好机会啦！(李六如《六十年的变迁》Ⅰ—145)Now that both the old Empress and Guang Xu are dead,it’s certainly a golden opportunity for us.
千载一时qiān zǎi yī shí
一千年才有这么一次机会。形容机会难得。golden, a rare opportunity, once in a blue moon