If you go to the theme parks section of the DIS, they have park times and fireworks schedules. The schedules for September are already out. Not to burst your bubble, but there are nights in September that have no fireworks. If they are scheduled for 8 or 9 pm you should be fine. For 9 pm just hang out a bit if need be in the lounge and head for the obervation deck a few minutes before 9.