that'll be the day meaning
- [American idiom]
I don't believe that the day will ever come (when something will happen).
• Do you really think that John will pass geometry? That'll be the day.
• John graduate? 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!"