My girlfriend's birthday is coming up, and I'm gonna ruin it.
That's horrible! Why would you want to ruin her birthday?
Because I want to make her day miserable in front of all of her friends.
It is wrong to ruin someone's special day.  It is especially disturbing if it is your own girlfriend.
It's only her birthday. It's not like I'm going to ruin other important days to her.
Intentionally ruining any day of your girlfriend is unacceptable, but it's especially bad to ruin her birthday. You need to treat your girlfriend with love and respect and support her on her important days like her birthday! If you can't do that, then I don't think you're ready for a relationship.