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 ."