Definition
Used to express disbelief or skepticism about something that is said to be going to happen; indicating that the speaker thinks the event is highly unlikely.
（表示不相信）哪有的事
"She said she'll be on time tomorrow." "That'll be the day!"
「她说她明天会准时。」「那才怪呢！」
"He promised he'd pay me back the money." "That'll be the day!"
「他答应会还我钱。」「那才怪呢！」
"They said the company wouldn't lay off employees." "That'll be the day!"
「他们说公司不会裁员。」「那才怪呢！」