that'll be the day
used to say that you think something is very unlikely to happen

[TahlilGaran] Dictionary of Contemporary English ▲

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

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

[TahlilGaran] English Idioms Dictionary ▲

That'll be the day!
something that you say in order to show you think an event or action is not likely to happen.
A pay rise? That'll be the day!

[TahlilGaran] English Idioms Dictionary ▲

that'll be the day
informal That will never happen.
Joe wanted me to lend him money to take my girl to the movies. That'll be the day!
"Wouldn't it be nice if we had to go to school only one day a week?" "That'll be the day!"

[TahlilGaran] English Idioms Dictionary ▲