that'll be the day

/ˌðætl̩ bi ðə ˈdeɪ/

UK: /ˌðætl̩ biː ðə ˈdeɪ/

that'll be the day

English Phrase

Definition

Said in reply to something that one believes will never happen.

Originally referring to a significant day in the future regarded as worth waiting for.

"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."