that will 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 will be the day

something is very unlikely to happen. informal
1991 Alistair Campbell Sidewinder ‘Now for my proposal, which you'll find irresistible.’ ‘That'll be the day.’
See also: that, will
Farlex Partner Idioms Dictionary
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