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.

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.

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.

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

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

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

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
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
Now in its 33rd year of consecutive touring of national touring, That'll Be The Day is living proof that Rock 'n' Roll will never die!
Rock 'n' roll magic at the Town Hall
Now entering its third year of collecting, That'll Be The Day is delighted to have raised over PS200,000 in donations from its truly generous audiences across the UK!
That'll Be The Day to make your night
Contributors to That'll be the Day include John Steel who would find fame with The Animals.
The 1950s, the decade when we were told we'd never had it so good..
"We first performed That'll Be The Day in Butlin's, Minehead.
What makes Trevor's day
Anderson's Tom Jones impersonation is just one part of the hit show That'll Be The Day, which is coming to Cardiff and Llandudno.
Arts: It's Anderson the Voice - even down to what's thrown at him; That'll Be The Day star serious about his impersonation of Welsh singer
That'll Be The Day is renowned for its work with worthy causes, and is currently supporting the Make-AWish Foundation UK.
A reason to rock out
Number 1 album THE original soundtrack to That'll Be The Day was spending its last week at no.
THIS WEEK IN 1973
Although aged just 22 when he died, his music has lived on in such classics as Peggy Sue, Oh Boy, Rave On, Heartbeat, Raining In My Heart and the aforementioned That'll be the day.
BEST SHOW
Jenna Jack, of the Spa Centre, says: "That'll Be The Day is a firm favourite here and always pulls in a large crowd.
WIN ...tickets to rock 'n' roll show That'll Be The Day
``I thought you were great in Godspell and That'll Be the Day and I've got all your records and you're so gorgeous and I can't believe I'm having lunch with you and I can't stop shaking oh this is the best day of my life .
JACI STEPHEN COLUMN: Cult of celebrity? Get out of here!; The Queen of the Columnists
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.
Curtain call: Rock history
He co-starred in That'll Be the Day (1973) as aTeddy Boy and appeared in The Last Waltz, the Martin Scorsese documentary film about the 1976 farewell concert of the Band.
He loves shoes, yeah, yeah, yeah ... TINA MILES AND DAWN COLLINSON... OUR GIRLS IN THE KNOW INSIDER GOSSIP RELAXED RINGO ROCKS OUT IN FOOTWEAR AD
BACK by huge demand, prepare for a party as the nations favourite rock 'n' roll variety show That'll Be The Day, returns with its brand new show!
Classic gold hits
The live production brings to life classic hits such as Peggy Sue, That'll be the Day and Oh Boy.
An evening for rock and roll lovers; A legend will be reborn at The Sage Gateshead this weekend. Entertainment Editor GORDON BARR has the details
Love Me A rock 'n' roll toe-tapper, co-written by Holly and Mitchell Parish, Love Me originally featured on the album That'll Be The Day.
FREE CD; DAILY Mirror LEGENDS THE LEGENDS COLLECTION GET THIS VOUCHER TODAY buddy holly