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

phrase

"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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