Great question, Happy Birthday in advance! You are going to have a fun night
So when I need a party outfit, I usually head to Forever 21. Do you like their stuff?
If you like to be comfy, you could pair this amazing top:
...with your fave dark-wash jeans and some black boots. Carry a black open-front drapy cardigan that you can toss on when you're cold, then bunch up in a ball and toss in your purse when you don't need it.
If you want something more girly, try this dress:
or this dress:
...wear with dark opaque tights, comfy black boots (like these) and top with a black blazer that you can keep on or take off depending on the situation.
Clutches are fun but you might forget it someplace, so I'd opt for a fun evening bag with a strap so it stays close by. You could always wrap your cardi or blazer kinda over the bag to keep hands free.
Dear readers: What did you wear to celebrate your last birthday?