Disney announced another 24-hour day from 6am on May 23rd to 6am on May 24th via Disney Parks Blog. I will let you consume the nonsense about hashtags and Disney Sides via their website. They do go into a bit of detail about costumes. I would only remind you that the average high in late May is 85 degrees with evening lows in the mid-60s.
Expect relatively light crowds from 6am on May 23rd through about 8pm on May 24th. The novelty to these 24-hour days is the late night when Magic Kingdom is never otherwise open past 3am. You’ll be able to get a ton done from 6am – 12pm. With Memorial Day Weekend crowds, the afternoon is going to be busy, but not unmanageable. Crowds will begin streaming in after 6pm and Magic Kingdom’s parking lot should close to capacity around 10pm with guests sent to the Epcot parking lot to monorail in. The late night has a unique, fun vibe, but it’s going to be extremely busy from 7pm – 3am in particular.