Bat's Day is the day that all the Gothic and Industrial "kid's" come out to
Disneyland for the day.
The event is held at the hotel on the Friday and Saturday and then everyone goes to the parks on Sunday. This means for the most part, the only day that crowds will be effected at the park is Sunday.
You will see lots and lots of "unusually" dressed people of all ages, all over the parks, and you will see lots of people at the Castle for an afternoon photo, lots of kid's and their parents for the Sword in the Stone show in FL and at approx. 8pm, almost every goth in the park out in front of the HM, for the head count and for photos.
Lots of people love the event for the people watching, and the group is one of the most liked unoffical events at the parks, because there has been so little trouble when they come to the park.
There is an offical event website
www.batsday.net if you want to know more.