that'll be the day

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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. © 2022 Farlex, Inc, all rights reserved.

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. © 2002 by The McGraw-Hill Companies, Inc.

that'll be the day

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]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

that'll be the day

COMMON People say that'll be the day to show that they think that something will never happen. `He says he's going to decorate his house.' — `Hah. That'll be the day.'
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

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 © Farlex 2017

ˈthat’ll be the day

(spoken, ironic) used for saying that something is unlikely: ‘When I’m rich, I’ll buy you a new car.’ ‘That’ll be the day!’
Farlex Partner Idioms Dictionary © Farlex 2017

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 Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.

that'll be the day

That is very unlikely to occur. Also put as that will be the day, this phrase, stated with an emphasis on that, dates from the early 1900s, and quickly spread throughout the English-speaking world. Eric Partridge pointed out it was so popular in South Africa that it even spawned an Afrikaans equivalent, dit sal die dag wees. A 1957 song by Buddy Holly was entitled “That’ll be the Day,” and the phrase also was used as the title of a 1973 British film.
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
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