I never understood the MegaCorp mandatory fun days. I got that speech about how I wasn't required to participate but that it was "strongly encouraged".
There was even a guy on our team who said he couldn't participate because he was trying to meet his deadline. Got the same speech from the director so he gave up on work and just endured the celebration. Later that night, after the event, he was playing poker in the office. Director wanted to know why he wasn't working
It's all a bunch of BS in my opinion. You can't force fun. If you like your coworkers, you already hang out with them. If you don't, making you attend a silly function isn't going to build team spirit.
I say you don't dress up and if they ask why tell them you are dressed up as a "party pooper". Or if you wanna take that to the next level, wear a 1$ birthday hat and carry around a
bag of poop
roll of toilet paper