that'll be the dayPROBABLY#used to say that you think something is very unlikely to happen 极不可能 ‘Bill says he’s going to start going to the gym.’ ‘That’ll be the day!’ “比尔说他要开始去健身房了。” “这不可能吧!”
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!''
→ that’ll be the day at day(SPOKEN SECT)