Each park is a little different.
At MK,
Guests are massed in front of the train station. The group is greeted by the Mayor, there is some singing, then the train arrives carrying the host family for the day. They are introduced, there is a countdown, confetti and the park is declared open as people run through the arches, down main street and to their favorite mountain (only to be slowed by CM's as they enter the individual lands...
At Studios
You are stopped just before the fork in the street leading to ToT and RnRC. There is some entertainment by Streetmosphere characters, a countdown then off you go.
For AK,
After collecting at the fork in front of the tree of life, there is a skit with Donald, Goofy, Minnie and Mickey welcoming you to the park and declaring it open.
Epcot you collect near Pin Central (at the split leading to The Land and Test Track. There is a brief welcome ceremony, a host family is introduced riding in a test track vehicle and the park is declared open.
Descriptions are not so exciting, but the events are nice to see, at least once. Some would not miss it on each visit.
