Definition
Said in reply to something that one believes will never happen.
Origins
Originally referring to a significant day in the future regarded as worth waiting for.
In Context
- "I’m going to get into work on time today. — That’ll be the day!"
- "Transmission from Klingon ship: Federation ship Enterprise. Surrender and prepare to be boarded. / Jean-Luc Picard (Patrick Stewart): That will be the day."