# that'll be the day

idiom

Source: https://englishreference.com/idiom/thatll-be-the-day
Updated: 2026-08-10

## Meaning

Said in reply to something that one believes will never happen.

## Examples

- "You'll tidy your room?" "That'll be the day."
- When my brother said he would stop playing video games, I laughed and said, "That'll be the day."
- She promised to arrive on time for once, and her skeptical colleague muttered under her breath, "That'll be the day."

## Origin

The expression originally referred to a significant day in the future regarded as worth waiting for. It is recorded as an idiom of anticipation, though its modern usage is more commonly reserved for events viewed as improbable.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.