While the events don't start until 9PM or later, you can get into the parks at 7PM on event night with your Disney After Hours tickets (no need for an additional park ticket). When you go into the park at 7PM, you'll go through a special entrance where you'll receive an After Hours event wristband. You have to show this throughout the night, but mostly just the first hour of the event or so, to prove that you're there for the event. Anyone without a wristband will not be allowed to ride or enjoy the event perks. If you're already in the parks before 7PM, there will be a designated location to get your wristband. I'm not 100% sure where it is in each park but if you ask a cast member, they'll be able to direct you.