Labor Day is on September 7 in 2015, which makes it a little tough to estimate what is going to happen in terms of HalloweenTime. If the pattern from previous years hold, HalloweenTime would start on Friday, September 11. However, given the significance of that date, it seems unlikely that DLR would opt to start HalloweenTime on 9/11. On the other hand, given the popularity of HalloweenTime, it seems unlikely that they'd wait until the 18th to start. So, I'm guess that the official start will be Saturday, September 12th.
Working off that assumption (which is a big assumption), it would mean that Space Mountain would go down on Tuesday (9/8) for the installation of the overlay. Haunted Mansion would still be closed for installation of the overlay (since mid/late August). Big Thunder Ranch would also be closed for installation of the Carnival overlay. All would likely re-open with the start of HalloweenTime or possibly on Friday (soft opening). Much of the Halloween decor will already be in place and/or starting to appear.
In terms of the operating hours, the weekends (Fri-Sun) and Labor Day Monday would have long hours and full entertainment, but the weekdays would have already gone to "off-season" hours (opening around 9 or 10 am and closing around 8 or 9 pm). Fantasmic! and fireworks would likely only be presented on weekends (Fri-Sun and Labor Day Monday). Now, all of this could change given the 60th anniversary activities, but going off the 2014 calendar, I would expect low-to-modest crowds on the weekdays because it's after the end of summer, but before the start of HalloweenTime.