From Longman Dictionary of Contemporary Englishthat’ll be the daythat’ll be the dayPROBABLY#used to say that you think something is very unlikely to happen ‘Bill says he’s going to start going to the gym.’ ‘That’ll be the day!’ → day
Examples from the Corpus
that’ll be the day• The day I call you Chief Rabbit, Hazel, that 'll be the day, that will!• ``Bill says he'll wash the dishes tonight.'' ``That'll be the day!'' 