Hi Alice!
You can get more information on the One More Disney Day promotion, including being able to enter a sweepstakes at the
One More Disney Day website. Unfortunately, however, I don't think there's a lot of detail with what's exactly going to happen in the Magic Kingdom that day on the website.
However, according to some unofficial sources that I have learned to trust in the past, attractions that normally run with limited hours, such as the Jungle Cruise and Tow Sawyer's Island, will close at their normal times. Many other attractions will run throughout the entire 24 hour period. There will also be additional entertainment offerings, but no details have come out. Table Service restaurants will extend their hours until midnight and at least one Quick Service (counter service) location will remain open the entire 24 hours. Transportation from Disney hotels will be available 24 hours as well.
Sorry I don't have any more details for you. I suggest checking the Times Guide when you enter the park that day. Have fun!