I don't want to celebrate my birthday at work today
What's wrong with celebrating your birthday at work?
I don't really like socializing with my coworkers.
It's not unusual to not like your coworkers, but you need to be social with them if you want to get promoted. Sharing your birthday with them is a good way to build relationships. Just bite your tongue and go along with it, maybe you'll even get a birthday cake!
Ugh, it's so hard to pretend to be nice to them.
But they're doing this all for you, so do you think you should keep up the act just a little longer and join in? Forcing yourself to socialize a bit might even help your case at promotion since they'll get to see who you are.