Congratulations! What terrific news!
I'm a brand new member, and this is my very first post. I hope you won't mind. I am jumping in because I'm a writer and have attended similar events, so I figured my opinion might be worth something even though I'm a YLF (and style) newbie!
Anyway, I love Angie's question, and also Di's suggestion. Academichic is great for ideas! And Debora's right; if you have a great pair of dark wash jeans, they would work perfectly, along with a top that you absolutely love. Obviously, if the weather is going to remain cool, you might need a jacket or sweater, as well, and ideally, the outfit would look equally good either way (with the jacket/sweater on or off).
When I'm reading my work in public—and especially when the occasion is celebratory, like this one—I like to choose a bright top. Something that makes me stand out a little bit, something that makes me feel happy. In my case, that often means some shade of red, but it could be any other bright colour that suits you or feels good.
On the other hand, you don't want to stand out TOO much, and in my opinion, solids generally work better than patterns for this purpose; you want people to be listening to your words, not feeling sea-sick from staring at your outfit.
You'll also want comfortable shoes. Some height won't hurt, but wedges might be better than a regular heel. And some stand-out but not-too-dressy jewelry would also be in order.
Here are a few more tips that don't fall into the "what to wear" category but might be useful anyway:
1. Ask how long you are allowed to read. Pick a short selection from your work that takes your listener somewhere (i.e. a scene that ends differently than it started) and—this is crucial—practice reading it aloud, and time yourself. Do not go over your allotted time. Even if others go over, it is better for you to go under than over.
2. Read more slowly than you think you need to. (This goes back to timing yourself, again.)
3. If you can, pick an alternate selection of the same length but a different mood, just in case. If the person ahead of you reads serious, you can whip out the funny bit. If the person ahead of you reads funny, you can be serious. Sometimes it's just not possible to pick two very different selections, but if you can, it's great insurance and helps keep people in the audience happy.
4. If you are allowed to read your whole piece, that is a lovely treat that won't always happen. So enjoy!
5. Find out (if you can) what the lighting will be like. Sometimes, believe it or not, organizers don't think to provide enough light to read by, especially at evening events. My hairdresser came up with a brilliant solution: bring an itty bitty book light in your bag, just in case!
6. Find out (if you can) what the sound situation will be. Microphones don't always work or aren't always set up, so you might have to project. Just do the best you can and try to be gracious if there are technical problems. It's obvious, but it helps if you remember in advance that glitches are all part of the territory.
7. Enjoy! This is your moment to shine!