If your girlfriend is mad at you and you want to tell her goodnight, it is important to approach the situation with care and sensitivity. You can start by acknowledging her feelings and apologizing for whatever it was that upset her. For example, you could say something like: "I know you're mad at me right now, and I'm sorry for whatever I did to upset you. I just wanted to say goodnight and let you know that I'm here for you if you want to talk more tomorrow."
Another way to tell your girlfriend goodnight when she is mad at you is to offer to make things right and to show that you are willing to take responsibility for your actions. You could say something like: "I know I messed up and I'm sorry. I just wanted to say goodnight and let you know that I'm willing to do whatever it takes to make things right. Can we talk more tomorrow and figure out a way to move forward?"
Overall, the key is to be sincere and genuine in your apology, and to show that you are willing to listen and to work through any issues that may be causing tension between you and your girlfriend. By taking responsibility for your actions and showing that you care about her feelings, you can hopefully start to repair the relationship and move forward in a positive way.