Hi Donegal,
Wow! I thought that we were lucky normally visiting for two weeks at a time but 23 days - I am so jealous!
I have checked the park calendar for Disney's Hollywood Studios on the Walt Disney World website for the duration of your visit and can tell you that
Fantasmic is currently scheduled to be shown on the following dates:-
2, 4, 7, 9, 11, 14, 16, 18 & 20 August 2011 at 9pm
21, 23 & 25 August 2011 at 8.30pm
Advanced dining reservations for the Fantasmic Dining Package can be made
HERE.
I hope that you enjoy the show and wish you a magical trip!