I’m short on time as I’m about to leave for a weekend stay at the Grand Floridian, but Disney released the September 2015 operating schedule at the usual link: http://media.disneywebcontent.com/StaticFiles/ParkHours/WDWTravelAgent_september.pdf. It looks like Mickey’s Not So Scary Halloween Party will start September 1st again. September 11th and 12th are Night of Joy dates. Also note the change in the Extra Magic Hours schedule beginning the week of September 20th to accommodate the start of the Food and Wine Festival on Friday September 25th.
The late labor day should not have a dramatic impact on summer crowds being extended into September. If you’re flexible with your dates, I’d visit between September 9th and 23rd, which should be a stretch of the two least crowded weeks of the year. But the first week in September should remain quiet, relatively speaking.