If it is your granddad's 90th birthday, suck it up and go.
You need to give your parents a gracious excuse they can offer the hosting family that will not embarrass them. Big school project? band concert? Out of town school thing? Scout event? Prior commitment, such as overnight or day out with another family?
If you are old enough you can have a bad period- that would allow recovery the next day.
However, part of having a family is pleasantly and graciously attending family events, so take a look at why you don't want to go, and consider spending your energy figuring out a way enjoying it instead of avoiding it. If there are others your age, maybe you can come up with an activity for al of you. If little kid thing, plan a game, craft activity or magic show. And so forth.
I have to go to a family party and I don't want to go! I can't say that I'm sick because people from the party are going coming over the next day so if I say I'm sick they'll see I'm not. Any suggestions?
