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!"
THAT'LL BE THE DAY
Meaning of THAT'LL BE THE DAY in English
American idioms English vocabulary. Английский словарь американских идиом. 2012