At MK and Ak they now have a morning opening show that is done before the park opens (not sure about what Epcot and MGM do). Unless you have an ADR for breakfast you will not be allowed in the park until the official park opening time. At MK for example, they will let you in the turnstiles but they will not let you through the tunnels under the train station. They hold everyone between the turnstiles and the train station. The morning show begins at 8:50am (7:50am on EMH mornings), the Mayor (or another citizen of Main Street) will come out on the train platform with singers and dancers and they do a welcome to the park, then the steam train comes in with the characters on it and the "opening family" and Mickey. Mickey, the Mayor and the opening family will do a countdown to open the park. Once the show is over they will allow you onto Main Street.