We were there a few years back. We had a great time and have considered going back for that week. There was a parade just about every day for the week leading up to 9/21. The fireworks occurred around midnight on 9/20 (within the first minutes of 9/21). It was a great party.
We had good weather. Keep in mind that Sept is the historical peak of the hurricane season. We have spent many Septs in the Caribbean and never encountered a hurricane there at that time. Just lucky that way. Once is Sept in Bermuda and twice in Nov in the Caribbean. No direct hits though. Another thing, a number of AC restaurants and resorts close 9/15 to 10/15. That didnt bother us at all. There were plenty of places open. We didnt miss a meal or a drink.