Hi April,
So awesome to see you are already planning for 2012!
This year the
Disney Parks Blog announced the 2011 Mickey's Not-So-Scary Halloween Party (MNSSHP) dates on May 2, 2011. One would guess (I don't know for sure) that the 2012 dates will be announced around that time next year as well.
If you get to your 180 window to make your Advanced Dining Reservations (ADRs) before they announce the 2012 MNSSHP dates, I'd go ahead and make my ADRs. If they conflict with the party dates, you can make changes. If the 2011 patterns hold true in 2012, MNSSHP is held on Tuesdays & Fridays in September beginning 9-13-11. In October 2011, looks like Wednesdays are a day off from MNSSHP.
Near the end of April 2012, I'd recommend to start keeping your eyes on the Disney Parks Blog. They usually make announcements there. If you're on twitter, follow @DisneyParks & @WaltDisneyWorld. If you're on Facebook, keep an eye on the official
Disney World Facebook page. You're also welcome to check back with us around that time.
Wishing you many magical memories :-) Allison