What does Bat's Day mean?

Bat's Day meaning in Urban Dictionary

Also known as 'Goth Day', Bat's Day could be the last saturday in August where goth children throughout the U.S and European countries flock to Disneyland for daily of princess boo-ing, high heels, and black lip stick. Although it's perhaps not officialy reconized by the park it self, the playground staff is well-aware of their existance.