What should I wear to my friend's birthday party?

You've got a ton of options here, but a safe play is to keep it casual. It's also probably the easiest option. If it's a daytime party, you can go with fitted jeans and a playful top. That way, you'll be comfortable, but you can show that it's a special occasion by adding a special touch with a nice top that isn't part of your everyday wear. If the party is mostly outdoors, flowy dresses with playful patterns are perfect for daytime fun. For a nighttime party, add some drama with dark jeans and an off-the-shoulder top. But be careful not to try dress more fancy or flashy than the person you're celebrating. You're a guest celebrating them, so you shouldn't steal their spotlight!