should i join my gf or go with my family to actually have fun

So i 22m and my girlfriend 19f are going out to spend time with her friends because they can't spend her birthday together on the day of her birthday so they decided to do it the day before for detail her birthday is tomorrow and we are doin this stuff today. I want to spend time with her on her birthday i dont really see the need to be here for her today seeing as how she is gonna be around all her friends and im just going to be there standing out but i digress my family has invited me to a movie that i have been wanting to watch and was planning on watching myself until these plans got in the way. I just want to know if im making the right choice by going with her instead of doing what i actually want to do thoughts?