Illuminations is not listed in the September schedule on the Disney website, but I can't find anything that says it will be down and why. Weird.
Since it is my most favorite thing in all of Disney World, I'm really glad that we're arriving in August this trip. For our arrival day, we have an early dinner ADR at Chefs de France so we can catch Remy while he's still there and hopefully see Illuminations that night.