I'm going to ask my roommate if his girlfriend could show up later in the week
Will you let her stay there if she comes earlier?
I'd rather her come later on, I do not want to see her early
I understand that you don't want your roommate's girlfriend in your home until later, but you can't tell your roommate how to live his life. Talk to him instead and try to compromise.
It's my house too. I have rights to say whatever the fuck I want to do with my house.
You have a right to voice your opinion, but you do not have a right to demand your roommate to do something. If he brings his girlfriend early, you should leave or deal with it.