that'll be the day

Phrase

idiomatic

(idiomatic) Said in reply to something that one believes will never happen.

Japanese Meaning

（皮肉を込めて）それが起こるはずがない、実現不可能な出来事に対する返答

When he said he'd finally learn to cook, I just laughed and said, 'that'll be the day.'

彼がついに料理を覚えると言ったとき、私はただ笑って『そんなことは絶対に起こらないだろう』と言った。

Sense（1）

(idiomatic) Said in reply to something that one believes will never happen.

I’m going to get into work on time today. — That’ll be the day!