I'm not going to be at the hospital for my dad the moment of heart surgery
Why would you not be there for your dad?
I can't stand my family and I'm going to party instead.
I understand you want to be at that party, but this situation sounds serious.  You should be there for a family member when they're having a procedure like that.  You wouldn't want to be left alone in a hospital, would you?
I can handle it and my dad is the asshole here.
It is situations like these where people don't understand the seriousness of their actions. If you skip this and something bad happens you would regret it for the rest of your life.