The commemoration of the nation's bicentennial went on for months. The United States Mint began issuing bicentennial coinage beginning in 1975 dated 1776-1976. That's why you can't find a 1975 dated Quarter.
Festivities included elaborate fireworks displays over major USA cities. Those in Washington, D.C were presided over by President Gerald Ford and televised nationally. A fleet of tall-masted sailing ships gathered in New York City for the occasion.
Many commercial products were marketed in packages tying them to the Bicentennial.