that will be the day meaning

• (informal)
    That is very unlikely

• by day:    [American slang]du ...
• by the day:    Adverb: by the day ...
• day:    Noun: day &nb ...

Examples

1. kevin ： that will be the day ! my boss is such a notorious penny pincher
2. That will be the day that I'll stop.
3. That will be the day that I stop.
4. Oh, that will be the day.
5. Maybe that will be the day after Christmas, but maybe it won't be until next year.