Every day is Halloween for some of the world's favorite rock stars, but when the real October holiday rolls around, some celebrities take full advantage of the opportunity to don outlandish Halloween costumes (we're looking at you, Mariah!). With the spooky day upon us, have a look at our gallery of famous folks in Halloween get-ups, from Christina Aguilera's naughty nurse to Beyonce as Spider-Woman to Elvis Presley's masquerade. Trick or treat!