They usually show the regular fireworks show, and then when it is finished they have a special tribute to america fireworks immediately afterward. Last year we arrived around 7 and went to the castle area and waited for the fireworks. There was no room to stand or walk and we were pretty far back. It gets really crowded. It usually starts to thin out a bit after the fireworks. The major crush of people is usually between 6 and 9. They usually reserve a whole level of the parking structure to be filled in from about 7 until 9. It generally takes 45 minutes or more to get into the parking structure at that time. In fact a lot of people will pay to get into the parking structure, go up to the top level and sit and watch the fireworks from there. I know my sister has done it a few times when she didn't get there early enough to actually get in the park (she arrived about 8:30 and didn't get parked until close to 9:15.
We go every year for 4th of July fireworks and New Years Eve. It is extremely crowded though.