On October 25, 1971 the Magic Kingdom hosted it's Official Grand Opening Parade at 2 PM. While the park had been opened since October 1st, the parade debuted 3 1/2 weeks later.
The Disney Parks Blog shared a video of the 1076 piece marching band that participated in this parade
The Official Disney Parks Blog shared this description of the parade:
First up, a small marching band dressed in red and white uniforms played “The Mickey Mouse Club March,” while Mickey Mouse kept the beat on an over-size drum. A large cast of Disney characters followed, including: Dumbo (“Dumbo”); Queen of Hearts and White Rabbit (“Alice In Wonderland”); Thumper and Flower (“Bambi”); Gepetto and Honest John (“Pinocchio”); Snow White, the Seven Dwarfs and the Queen (“Snow White and the Seven Dwarfs”); the Aristocats (“Aristocats”); and Winnie the Pooh, Heffalumps and friends (“Winnie the Pooh” featurettes). A fleet of marching toy soldiers followed, trailed by waltzing alligators and hippos from “Fantasia.”
A highlight of the parade took place when the Herald Trumpeters of the United States Army Band lined the top of the buildings on Main Street, U.S.A., to signal the arrival of a 1,076-member marching band. This line of marching musicians, directed by “The Music Man” composer Meredith Willson, stretched from the park’s main entrance all the way to Cinderella Castle and played “76 Trombones.” A mass choir then appeared on the castle stage to join the band in “When You Wish Upon A Star.”