Hello Helen!
I'm so glad that you chose to visit the Moms Panel with your question!
During late May, the Wishes Nighttime Spectacular fireworks at Magic Kingdom typically begin at 10pm, although on some occasions and particular evenings may be as early as 9pm (based on historical information).
To be safe, I'd recommend that you consider a late dinner at
Narcoossee's, booked for 8:30-8:45pm. Upon arrival, request to wait for a window-side table if you wish, or simply wait to view the fireworks from outside following your meal if you prefer. The Grand Floridian's grounds are lovely for an after-dinner stroll and watching from the shoreline is so serene.
I hope that this information assists you in your planning Helen.
If I may be of further assistance to you, I hope you'll consider writing me back directly. I love sharing tips and recommendations and would be happy to hear from you again!
Wishing you a simply Magical time as you plan for your May 2013 trip to Walt Disney World!
Kirsten