During the summer and peak holidays, they'll open at 08:30am for sure, but it's not written anywhere, just happens. The best plan is to show up between 08:00am and 08:15am. By 08:30am, a line will form that goes back to the ticket booths. If you are near the front of this line, you can get into the park and do a few rides with no wait. Even if you have UEPs it's a good idea to arrive early to get front seat rides on the coasters.