i am 4 days post op right now. Tomorrow is 5 days. I feel like crap. I am weak and tired. My belly is still swollen and i still have to have help when I stand up sometimes. The port area pulls a little bit when I stand. I can walk fine on my own and really not in any pain. However, I feel like I had a major bout of flu. That hit by a truck feeling has not left me yet. I would just wait and see how you feel. I am sure people will understand if you are not feeling up to a party. Also, depending on who's party it is, if you are feeling crappy you run the risk of taking away attention from the birthday boy/girl. If people are making sure you are ok they arent paying attention to the reason for the party.
Just above all listen to your body and take care not to over do it. If you feel up to it, go. But just make sure you are doing the best for you.