For anyone who thinks a full day at Disney isn't already a test of endurance, the Magic Kingdom will stay open 24 hours nonstop over Memorial Day weekend. The Magic Kingdom, along with Disneyland and Disney California Adventure, will stay open from 6 a.m. May 24 to 6 a.m. May 25. It's part of the hoopla surrounding the June 21 release of Disney Pixar's Monsters University. During the ''all-nighter,'' movie characters Mike and Sulley will be the grand marshals of the "Celebrate a Dream Come True'' day parade and make appearances in Tomorrowland. Characters will dress in their PJs, and the Cinderella Castle will host late-night dance parties. Only select attractions, entertainment and services will be available.