That’ll be the day!

that'll be the day

Said of something that is unlikely to happen. A politician speaking the truth? Ha, that'll be the day!
Farlex Dictionary of Idioms.

That’ll be the day!

exclam. That will never happen! When he gets his own car—that’ll be the day!
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • on one's
  • on someone's
  • #dead
  • (Have you) been OK?
  • other than
  • other than (something)
  • pillow-biter
  • out of one's
  • let something drop
  • (I've) got to go