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!

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!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
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
References in periodicals archive
That'll be the Day! 1950s Newcastle, through Tyne Bridge Publishing, is on sale now.
That'll be the Day! 1950s |Newcastle, through Tyne Bridge Publishing, is on sale now, priced PS13.
The 1950s, the decade when we were told we'd never had it so good..