that'll be the day
Said in reply to something that one believes will never happen.
Meaning adapted from Wiktionary, available under CC BY-SA 4.0.
“"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."”
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.
- 01
a cold day in July
The time of occurrence of an event that will never happen.
- 02
a day late and a dollar short
Too late and too feeble to achieve the desired effect.
- 03
a year and a day
A long time.
- 04
all one's born days
One's whole life.
- 05
as the day is long
Unceasingly; very; thoroughly; to a very high degree.
- 06
bad hair day
A day when things seem to be unmanageable or go wrong.
- 07
bad old days
The past, seen as a time when things were worse.
- 08
banyan day
In British naval tradition, a day of the week when galley kitchens served no meat on board ship.
Last updated August 10, 2026