They were emerging from Reconstruction, and there was the successes of science, industry, and cultural exchange on an unprecedented level. There was everything from a giant Corliss steam engine and Turkish scarf dancers, to hundreds of replicas of fishes. [link]
The idea of the Centennial Exposition is credited to John L. Campbell. In December 1866, Campbell first suggested to Philadelphia’s mayor, William Strumberg Stokley, that the Centennial be celebrated with an exposition in there. It was, after all, the place the Declaration was signed.
Celebrations started as early as May 10, 1876 - there was a lot to see and do, had to start early.
Philadelphia was astir with the excitement of anticipation as a whole nation,
well prepared by months of publicity, waited. The day was May 10, 1876, and in a
few hours the President of the United States and the Emperor of Brazil would
open in Fairmount Park the great International Exhibition to celebrate the
centennial year of American independence.
Pennsylvania Historical and Museum Commission