that will be the day—
1. that will be a great day worth waiting for:
- That will be the day when every class is a place where every child has consistent, quality instruction.
2. (ironic) that’s very unlikely (is used to express doubt following some boast or claim by a person):
- “He says he’ll do the washing-up.” “That’ll be the day!”
those were the days—is used as an expression of nostalgia for a happier time past:
- Well, those were the days. It was a great time for jazz in Chicago.