I'm going to tell my girlfriend she can't go out with her friends on our anniversary.
How do you think she will react to that?
I am not sure, but it is important to me.
It's understandable that you would be upset at your girlfriend for not wanting to celebrate your anniversary as it's an important event for every couple and it's expected that you celebrate it. You should talk with your girlfriend and explain to her that the date is important for you and she can see her friends some other time.
Okay, thanks for the tip.
Don't come into the conversation super heated and angry at her. Just explain your side as calmly as you can, and I am sure she will understand your point.