In 1995 the celebration was at around noon. In 2005 I believe it was at 10am. Also in 2005, there was a broadcast of Walt's opening speech at 4:45pm, the exact time he gave the speech. The speech was shown on giant screens throughout the park.
The event takes place on a stage in front of SB Castle. The 50th featured several Disney Execs, including (I believe) Eisner and Iger. Diane Disney Miller spoke, as well as Gov. Schwarzenegger. Art Linkletter was the emcee. The 40th event was similar, except they unveiled the "Time Castle" that would be buried in front of the castle. Singer Randy Travis also sang "Happy Birthday" from atop the Matterhorn. Of course the characters were on hand, and there were some daytime fireworks.
This year I'm sure the castle celebration will be similar.