I'm not going to attend my friend's 40th birthday
Is there a reason that you want to avoid the party?
Yes, several months ago he threw a party, and I was locked outside since midnight. I left a note on his front door - good night!
I guess it's ok if you don't want to attend their birthday. Just make sure you tell them as it'd be convenient for them to make  proper arrangements.
He's boozing and partying too much. Last time he threw a party, people were throwing up in his house, and other bad stuff. I don't want to associate with those kinds of people.
It sounds like you have a whole slew of reasons that you do not want to attend and celebrate. It is okay to not always be in the mood to celebrate.