Why has my manager asked me to come in for a 'private meeting' on my day off? She's refusing to say what it is about. Am I getting dismissed? Should I just hand in my notice before I get fired?

I don’t know anything about you. In fact, I know more about your manager than you since you’ve described her as female. Maybe she’s being kinky and she’s got the hots for you and she’s taking advantage of this opportunity. But what does your actual contract say? It does seem odd that she’s refusing to give a reason and that the meeting is private. Most decent employers would give a reason and would also ensure that at least your attendance would be recorded so that you would be paid of compensated for your time. Most companies would also allow another colleague to be present as a witness if you’re uncomfortable or believe it to be necessary. Can you think of any reason for your manager to get rid of you? But never hand in your notice unless you know that you’re the offending party as it’s the perfect excuse for them not to offer any redundancy or severance pay. If you know you’re the offending party, it might be easier to exit as fast as possible. But you never know. It could be that someone else in in for it and that she wants you to be the first in line for a promotion.