E ticket night is when the MK is open 3 hours later for resort guests with multiday passes. You do have to pay extra for it... $12 for adults and $10 for kids 3-9 years old, but the tickets are limited in number, though few sell out. They have the headliner rides open at that time, which include:
Splash Mountain, Space Mountain, Big Thunder Mountain Railroad, Country Bear Jambooree, Buzz Lightyear, Alien Encounter, TTA, Starjets, and the Haunted Mansion. Sometimes Pirates of the Carribbean is open, too.
The nights will be listed probably 2 weeks prior on the offcial website. Otherwise you can find out once you get to your resort to check in. I believe you can only purchase the tickets 5 days in advance, so you may want to purchase them at your resort, but you can purchase them at guest relations inside the MK, too . You will need to show your proof of resort and ticket media to purchase the tickets, and also when you exchange your ticket at the MK guest relations for your wrist band, which allows you to stay in the parks and ride after the park is closed that night.
Hope you enjoy it. We always do!