That will be the day! 会有那么一天吗？

That will be the day是一个用得非常普遍的习惯用语。That will be the day美国人用这个习惯用语已经有四十多年了。还带有浓厚的讽刺性。

Sure, the boss promised us a raise again, but he's done that before and then finds some excuse not to do it. This time he said we can expect a nice raise by January 1. Yeah, sure - that will be the day！ We're afraid it just won't come true!

是呀，老板又向我们保证要给我们加工资。可是，他以前也这么保证过，然后又找借口不给加。这次，他说，到一月一号，我们就可以加不少工资。 哼，谁信他的！恐怕这根本不会实现。