That’ll be the day

phrase

Used to express disbelief that something will ever happen.

"They said they would clean up after the party. That'll be the day!"

Informal

Idiomatic, Colloquial

Definition 1 of 1

Pro Tip 1/3

Shows strong doubt or sarcasm about the likelihood of something happening.

He says he's going to start his own business. That'll be the day!

Pro Tip 2/3

Common in English-speaking countries to humorously dismiss overly optimistic statements.

You're going to do all your homework by Friday? That'll be the day!

Pro Tip 3/3

Not meant to be taken literally; it's used to emphasize skepticism, not to predict an event.

They're going to stop arguing and work together? That'll be the day.

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