Hi Rachel,
How incredibly cool that you will be at Disney World for the kickoff of
Summer Nightastic! I'm jealous but hope you have an awesome time!
The Move It! Shake It! Celebrate It! Street Party is usually held 2 or 3 times a day at Magic Kingdom. Currently, the on-line information I see only goes through May 14, 2010 and shows it being held daily at 11 a.m., 12:30 p.m. and 5:40 p.m. They usually only publish the schedule a couple of weeks in advance for this Street Party.
You could call the events and entertainment line at 407-824-4321 to see if they have more information. If not, please feel free to check with me a couple of weeks before your trip and I'll look again for you.
Always pick up a Times Guide when you enter the parks as on-line schedules are always subject to change without notice.
Wishing you many magical memories :-)