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]
|a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.|
|an extraordinary or unusual thing, person, or event; an exceptional example or instance.|
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