Here are some of the dates that have been released for special events in 2012. Some of these are pretty far out so be sure to check with Disney as the events get closer to be sure.
Flower and Garden Festival 2012 – Epcot
March 7 – May 20, 2012
Star Wars Weekends 2012 – Hollywood Studios
May 18-20 and 25-27 and June 1-3 and 8-10, 2012
Night of Joy -Magic Kingdom
September 7 -8, 2012 (requires a separate ticket)
Food and Wine Festival – Epcot
September 28-November 11, 2012
You can get lots of great information about these events at here.