I don't mind which day I have off. Any day would be fine with me. These two answers are acceptable. "I don't care" is a little casual.

Since you are talking to your boss, you will have to be a bit formal. "I don't care which day'" or 'It doesn't matter which day" both don't sound formal nor decent enough. You could say instead ( there are many ways to put it ): "I'll leave you the choice, for me all days are good." " I am flexible with my schedule, I think it allows me to choose any of those days, you can suggest instead which day I could take off ."