We were there from May 17 to 25. First 3 days (mon-wed) were pretty quiet, but it was drizzly and cold, so perhaps that kept crowd levels down. Missed thurs and fri, went again from sat thru tues. Weekends were nuts - lines weren't unmanageable - we collected fp's and used them wisely, rode FL rides early AM, attended shows and shopped in the later morning and skipped to DCA after lunch, which always feels less crowded because the walkways are so much wider. Our last 4 days it seemed not many people were riding, but they were all blocking the walkways! Stay away from the path thru adventureland past Indy - got stuck in the shuffle for about 15 minutes one day! Probably had something to do with 3 kids in tow and a double stroller - not as maneuverable as those on foot! People started parking out for the Celebrate parade about 2-3 hours beforehand - 1 hour before, you would be hard pressed to find somewhere to sit and watch. Fireworks were the same - benches were "saved" from about 6pm.
On Monday, Screamin was out of FP's by about 2pm, but most other DCA FP rides didn't even have the FP machines running - GRR was 15 mins at the most, but if you kept checking back, you'd easily find a time when the wait was only 5 mins. TSMM was ALWAYS busy - never saw it when it didn't have a 40+ minute wait.
Hope that helps a bit - if you have any other questions, just let me know - right now everything is still pretty fresh in my mind....