Verbal sparring: "A bad day" is not an excuse to behave badly.

You don't get to treat people poorly because you're having "a bad day."

A bad day may cause you to be more emotional or temperamental than normal, but it doesn't mean that you're allowed to be rude, disrespectful, deceitful, unprofessional, mean, or abusive.

That is not a thing.

In fact, one of the best ways to judge a person's character is to observe their behavior when they are having a bad day. Anyone can pretend to be a decent person when the sun is shining and the bird are signing. The true test of character is to continue to act like a decent human being when nothing seems to be going right.  

In short, "I'm sorry. I'm having a bad day," is not an excuse for acting like a jackass. If someone attempts to excuse their behavior for this reason, push back on this nonsense. 

Push back hard.