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!

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!

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]

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.'

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

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!’

That’ll be the day!

exclam. That will never happen! When he gets his own car—that’ll be the day!

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.
References in periodicals archive ?
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The live production brings to life classic hits such as Peggy Sue, That'll be the Day and Oh Boy.
ROCK `n' roll's journey through the 1950s,60s and 70s makes for an entertaining two and a half hour musical called That'll Be The Day.