Unless it's a scheduled concert or filming session (both of which could probably be found on either an artist website or the official WDW website), I highly doubt that these celebrities just announce when they're going on vacation. They're probably tired of the paparazzi and just want to get away, so they probably aren't going to say "Hey, I'm going to WDW! Here are my dates and my reservations!". Usually people see them just by chance. I've seen several stars ranging from Rosie O'Donnell (back the year the Animal Kingdom opened, she was there for the opening of Tarzan Rocks which has since closed) and Whoopi Goldberg (no idea why she was there, really, but she was). A sure-bet time to catch stars that you like (like the Jonas Brothers and Miley Cyrus) would probably be the Christmas parade taping weekend (usually the first weekend in December). The downside is you usually need to be at the park (the MK) several hours before opening to be one of the first ones in and therefore near any of the real action. I've been in the park on taping weekend and it is packed. It's wall to wall, especially on Mainstreet and near the castle.
Other than that, a lot of stars are there on major holiday weeks and on special weeks. The last weekend of February is ESPN weekend and I'm sure there'll be some sports stars down there for that. Star Wars weekend would probably draw in some celebs. If you like Soap Operas, then Super Soaps Weekend would be the time to hit up MGM.
Ashleigh
