say in other words

[snade17]

Could you paraphrase the underlined part in the following:

Britt Blackerby, his attorney, had tried one last time to talk him out of it. They’d been friends for years; six months ago, when Paul first brought up all that he wanted to do, Britt thought Paul was kidding and laughed aloud, saying, “That’ll be the day.” Only when he’d looked across the table at the face of his friend had he realized Paul was serious.

[Gillnetter]

Could you paraphrase the underlined part in the following:

Britt Blackerby, his attorney, had tried one last time to talk him out of it. They’d been friends for years; six months ago, when Paul first brought up all that he wanted to do, Britt thought Paul was kidding and laughed aloud, saying, “That’ll be the day.” Only when he’d looked across the table at the face of his friend had he realized Paul was serious.

That will be the day, in this context, means that the probability of something happening is near zero. "His mother-in-law will accept him. Well, that'll be the day." Which means that his mother-in-law will never accept him.