The holidays are normally quite crowded. Federal holidays that fall on Mondays, but are not observed by everyone (President's Day, Columbus Day, etc.) are also quite busy, including the weekend before. The summer months are traditionally hectic, with the exception of early June and mid to late August. Spring break is another busy time, but that varies by school and when Easter falls - any time between mid-February to mid-April.
Least crowded times, IMHO - January (except MLK weekend), early February, May (except for Memorial weekend), early June, late August, November before Thanksgiving and the first two weeks in December. We have visited during all of these times and crowds are manageable. Of course, with extra celebrations planned for the 50th anniversary, that may not hold true.