On 9/11 as soon as it was ascertained that the country was under attack, WDW decided to close its parks immediately.
PAs played a very calm message that gave no inkling as to the horrors of that day just that due to circumstances the given park (they didn't say all parks as from what I was told that might have set off panic) would be closing. There was just a hard close, but no evacuations or anything of that ilk.
I can only imagine how bizarre it must have been, although you can be sure many people were getting calls/texts (did people text back then? I didn't) telling them what was going on.
I do know I visited two weeks later and it was eerie in a way I never would want to experience again.
Imagine being at The Mara at DAK Lodge at 6 p.m. and you are the ONLY guest in there for 40 minutes ... with a good eight CMs waiting on you and bringing extra food and free dessert. It was surreal ... not in a good way. The parks were just empty ... I have never seen EPCOT like that ... you could walk for stretches and not see another guest.
But then on Saturday the MK was packed as locals decided they weren't going to let madmen prevent them from living.
~Did we ever get Osama?~