As PP said, unless you have a regular or hopper ticket, you cannot get in until 4pm. But, the party does not get started until 7pm, so an adr sometime between 4pm-6pm would give you plenty of time to eat and still not miss any of the party.
We went last weekend and had a 5:15adr at Liberty Tree. We had a great meal and were finished in time to make it to the judges tent to see mickey and minnie by 7pm.