That’ll be the day
Definition 1 of 1
Pro Tip 1/3Sarcasm Alert
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 2/3Cultural Usage
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 3/3Not Literal
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.