What does rue the day mean?

Answer 1

It means that you'll regret something you did on that specific day, and that in the future you'll look back on that specific day in regret for whatever you did. So another way to put it, is " regret the day"

Answer 2

It basically means that you will regret what you did today. It's often used as a sort of threat, meaning, I will make you pay for this, and you will wish you had never done this.

Answer 3

mean