Bats Days is a non-official event that takes place over a few days at Disneyland during which people who identify a Goth come to Disneyland. It started in the '90s sometime and has grown and grown over the years. The park does not close early. It is not an official Disneyland event. You can count on LOTS and LOTS and LOTS of people there dressed in black and other Goth styles with piercings, tattoos, etc.... It's an amazing few days for people watching.
You can search online for more, including pictures. You will find lots of sites, articles, etc.... I am not posting links because I don't want to worry about something not ok to be on DIS being on a site I link.
It is also only on Sunday 5/2 in the parks. All of the rest of the activities will be held at a hotel near the parks on the other days.
The only thing that can get crazy is the afternoon photo in front of the castle and the approx 8pm photo at the manison. This will not block anything to you but it does create a lot of traffic in these areas for a bit before they occur.
And that is why they moved the event from the traditional August dates. It was ridiculously hot to be so dressed up The future dates should put it the first weekend of May, which stinks for Renaissance Faire People, but for weather should be awesome!