Inf. It will be an unusually amazing day when that happens! Bill: I think I'll fix that lamp now. Andy: When you finally get around to fixing that lamp, that'll be the day! Sue: I'm going to get this place organized once and for all! Alice: That'll be the day!
That'll be the day!
something that you say in order to show you think an event or action is not likely to happen A pay rise? That'll be the day!