Act like you have some desireable social graces that would make people want to be friends with you, show up to the party to make your friends happy, and try to like like you mean it when you smile\. And smile a lot to these people who love you for no apparent reason will think they made you happy.

The party is for THEM,. not you.

Email/text your friends and family that you will be out of town for your birthday.

You're an adult, explain to your family 👪 you don't wish to celebrate your birthday.

you're 40, quit Yahoo Answers

I'd tell them you made plans out of town. That way it gets canceled, and you have an excuse for not going. Just make sure, you're not anywhere where someone might run into you.

You're not alone. I hate celebrating my birthday.

Then plan something you would rather do.

I went white water rafting with someone for their 40th... it was super fun.

Go see a play.

Have a cooking class or paint class with your besties.

etc. etc