that'll be the day

Originally referring to a significant day in the future regarded as worth waiting for.^[1]

• (Received Pronunciation) IPA^(key): /ˌðætl̩ biː ðə ˈdeɪ/
• (General American) IPA^(key): /ˌðætl̩ bi ðə ˈdeɪ/

• Audio (General Australian):

  (file)

that'll be the day

1. (originally Australia, New Zealand, idiomatic, informal) Said in reply to something that one believes will never happen.

   Synonym: that'll be the frosty Friday

   I’m going to get into work on time today. — That’ll be the day!

   • 1990, Trent Christopher Ganino, Eric A. Stillwell, “Yesterday's Enterprise”, in Star Trek: The Next Generation, season 3, episode 15, spoken by Jean-Luc Picard (Patrick Stewart):

     Transmission from Klingon ship: Federation ship Enterprise. Surrender and prepare to be boarded. / Jean-Luc Picard (Patrick Stewart): That will be the day.

• a cold day in hell
• a week from next Tuesday
• until the cows come home
• when hell freezes over
• when pigs fly