that'll be the day

Idioms  

1. 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]

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Example Sentences

In one scene set in the 1950s, a diner owner scoffed, “A colored mayor? That’ll be the day.”

From Los Angeles Times

“That’ll Be The Day” was co-written by Mr. Allison and Holly after they heard John Wayne utter the title phrase in the 1956 western “The Searchers.”

From Washington Post

Then, in May 1956, he and Mr. Holly went to see a new John Wayne movie, “The Searchers,” in which one of Mr. Wayne’s most memorable lines was “That’ll be the day.”

From New York Times

Days later, according to an account written for the Library of Congress, Mr. Holly suggested that he and Mr. Allison write a song together, and Mr. Allison, imitating the Wayne line, said, “That’ll be the day.”

From New York Times

The two wrote numerous hits together as teenagers, including “That’ll Be the Day,” inspired by a line from John Wayne in the classic Western “The Searchers.”

From Seattle Times