We are staying at Fort Wilderness for both of the 2 weeks you are asking about. From what I have read the crowds are expected to kick in by the Thurs or Fri (17th, 18th). Since we are heading out in the wee hours of the 19th (we are 2 12hr days drive from home), we are thinking to leave the last day to kick back and enjoy the campground, thereby avoiding any increasing crowds in the parks. While I've never been there the entire President's Day weekend before, always before, it sure sounds like it is one of the peak times and I'd expect long lines.
When we first started planning we were only going for 1 week and we chose the first week for the lower crowds....
Annette