The only time you can wear a wedding dress in is the Trick or Treat party, but it has to meet the minimum guidelines - I put the ones from the web site at the bottom of this post.... Like Califgirl said, it's a Halloween party in DCA - not
Disneyland - and they have costume guidelines for reference.
That being said, I hate to hijack a thread, but I realize this is so special that I feel completely compelled to share....
I am one of the VERY few brides ever allowed into the park during operating hours - without paying for an in-park wedding.

It was back Christmas of 2001 and we had just gotten married at the GCH. I just happened to have a friend who talked to a friend who talked to a friend and pulled some strings..... I had to keep my leather coat on and my train bustled - which was fine by me - I was cold! It didn't keep people from realizing what I was wearing. With the hair and the tux and gown entourage of 8 - it was pretty obvious!
I do have a rather dark souvenier photo of us in front of the castle with my jacket off - call it a hit-and-run photo-op!

I had to have my piece of proof!
If I can dig it out of the album (we didn't go digital until late 2001), I'll scan it in and upload it for you guys tomorrow, while the thread is still fresh.