English version

that’ll be the day

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 dayThe 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!''
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