Hi Kiera,
I know exactly how you feel!
It is not very often that we host an event like this here in the UK and how typical that you will actually be at Walt Disney World on the very day that Rapunzels Royal Celebration will be taking place.
Details of the Worldwide Celebration to Honor Rapunzel, 10th Member of the Disney Princess Royal Court was first announced on the Disney Parks Blog on 14 June 2011
HERE
No further details have been provided as of yet but I suspect that this will also be done via the Disney Parks Blog.
I would therefore recommend that you subscribe to the Disney Parks Blog by clicking on the Orange tab at the top right hand side of the page which I have provided the link to above. This will ensure that once further details are announced, you will receive the information straight away.
Typically information about similar happenings at Walt Disney World is not published until nearer the time. For example, nothing has been announced as of yet about the 40th Anniversary of Walt Disney World which will be taking place the day before on 1 October 2011.
I hope that this information has been of help and will keep my fingers crossed that some sort of celebration will be held at the Magic Kingdom on 2 October 2011 that your family can attend.
Wishing you a magical holiday!