{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
Slang English vocab. Английский сленговый словарь. 2012