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3. that'll be the day

that'll be the day

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that'll be the day

(spoken)

it will never happen A raise in pay? That'll be the day!

Usage notes: said esp. to emphasize something you wish would happen

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]