that'll be the day

That will never happen, that's very unlikely, as in You think I'll win the lottery? That'll be the day! Presumably this phrase is short for that will be the day worth waiting for, but it is nearly always used ironically, as in the example. [Mid-1900s]

Words nearby that'll be the day

that ain't hay, that does it, that is, that makes two of us, that will do, that'll be the day, that's, that's _____ for you, that's about the size of it., that's how the ball bounces, that's one on me