The Haunted Mansion will re-open as Haunted Mansion Holiday with the start of HalloweenTime. If the past is a good indicator of the future, HalloweenTime would start on Friday, September 11th this year, but given the significance of that date, it seems unlikely that DLR would opt to start a major "season" then. So the best guessing is that they'll wait a day to official start HalloweenTime on September 12 (or thereabout). Given the popularity of HalloweenTime, it seems unlikely that they'd give up an entire week (and more importantly, weekend) of HalloweenTime to wait until September 18th to start, but somewhere between the 12th and 18th is the most likely period this year. Although, it is worth noting that with the 60th Anniversary, it is possible that everything will be different this year.