Try out one of these ideas instead. Chances are you have most of what you need for these costumes around the house and can run to a department type store to fill in the gaps.
You need: black v-neck tee, green pants, black boots (ideally lace up), and enough hair to pull into a side braid. That's it. Bonus points if you have a Mockingjay pin, a black rain jacket, or a quiver, bow, and arrows. You can also wear a marathon type sign that has "12" on it. This can easily be switched up to be a guy's costume and you could be from any district!
I've seen tons of superhero dress designs this year, and though I adore the evening gown styles, you probably can't pull that off at the last minute unless you have an old prom dress of the right color in your closet or feel like sifting through racks at thrift stores tonight.
Instead, look for any solid color dresses you have in your closet. If you have a green one for instance, you can add some gold accessories and become Loki. Then apply some lovely eye make-up (pictured above) to complete the look. Jangsara has tutorials for lots of superhero pallettes and even some Star Wars characters. If you have a Loki neclace, like this one for example, wear it with the dress!
Inspiration for this costume idea is from Janna O'Shea who has worn lovely Lady Punisher and Classy Marvel Girl costumes and from Jill Pantozzi - she formed the Classy Lantern Corps.
You'll need: a suit and blue rubber gloves. Go forth and be creepy.
If you like toys and collectibles, chances are you have a mask or helmet of some kind. I have a Boba Fett helmet for example. Take whatever you have - Captain America hood, Vader helmet, etc - and pair it with something unlikely. Go with pimp Vader or Fett like above, pair a Stormtrooper helmet with your renfaire gear, or just choose randomly from your closet. The results will probably be funny, and the only uncomfortable part of your costume will be the mask.
Why not mix Harry Potter and The Hunger Games? Don glasses, paint a scar on your forehead, and carry around a baguette. You could wear a Harry Potter-esque school uniform or jeans, t-shirt, and an apron.
Unless you have a Spider-Man or Superman spandex outfit lying around, being a superhero is out. But! You can be their alter egos. Dress like Peter Parker, Selina Kyle, Steve Rogers, Clark Kent, Barbara Gordon, Matt Murdock, Hal Jordan, and so many more. The best part about this is there are tons of comics, movies, and TV shows to pull references from!
You'll need: a cord with a washable black marker to wear around your neck and you'll have to draw hash marks all over your visible skin (don't use a Sharpie). It's simple and spooky. Bonus points if you get a red wig to look like Amy Pond.
This one might be a bit trickier to scrounge up, but I bet you have the basics for the BSG BDUs: gray tank, black tank, green pants, black boots. If you have dog tags (even if they aren't BSG ones), go ahead and wear them to help sell the look.
The Scooby Gang represents a wide range of the fashion spectrum. You can go from the trendy Buffy to the more comfortable choices of Willow. Dudes can rock the Giles librarian look or mostly-in-black Spike. This option is more fun if you can get friends to join in and make it a group costume.
Hopefully this gives you some ideas so that you aren't costume-less tomorrow! Keep in mind that you can stash these ideas for last minute cosplay for conventions, too.