Kings Dominion just released the calendar for 2013! Opening day is Friday, March 29th. It's just about a week earlier than what it was last year.
Link: http://www.kingsdominion.com/hours-directions/kings-dominion
The only major change I see for the 2013 season is the park is not opening until 12 noon on the Sundays in September and October. Same Sunday park closing times as it was in the 2012 season though (including haunt weekends).