As others have said, from the 16th-19th, you'll have moderate crowds. To me, that is a LITTLE less than summer crowds, but still very crowded. Yes, most schools are still in that week, but with year-round schools getting more popular along with homeschooling, AND parents who aren't afraid to take their kids out that last week before Christmas (because not much happens that week education-wise), it is a popular week to go. AND free dining is still in effect, so that draws in some people too.
Now the 20th-22nd will be CRAZY busy as that begins Christmas week. This is the number one busiest week of the entire year.
Just plan on getting as much of your must-dos done the first part of your vacation, and then do some fun stuff on those crazy days.