english
1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): That’ll be the day! 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!
McGrawhill's American Idioms And Phrasal Verbs