First, here are the Park Hours and Entertainment Schedule for September of last year, so you can get a sense of what was on the calendar:
http://www.wdwinfo.com/disneyland/park-hours.cfm?month=9&year=2010
Here is the Halloween at DLR Superthread - Part 2 (Pages 1 & 2 have tons of information on the Halloween Time season, but not specifically about rehabs):
http://www.disboards.com/showthread.php?t=2560082
The end of September will be part of the official Halloween Time season at Disneyland Resort. We're not sure of the start date of the season yet, but it definitely will have started by 9/27.
The end of September is generally a less crowded point at DLR, and a less crowded point in the Halloween season (in comparison with October, when the crowds pick up a bit). The hours will be shorter as a rule.
You will get to see Haunted Mansion Holiday and likely Space Mountain Ghost Galaxy, among the other festive Halloween highlights (detailed in the Halloween thread linked above).