The Great C to C-flat Transposition happened in an alternate reality where the Guild of Composers had, for several centuries, been continuously requiring higher pitches and higher tunings in their music. The effects resulted in tensely strung (though brilliant sounding) stringed instruments; but the woodwinds and brass were much less pleased, as they had to be content with lopping bits off their horns in order to tune properly.
One day, in the Summer of 1566, the Chancellor of Musical Arts summoned the Guild chiefs from the various instrument makers and the composers Guilds. Boxes full of chopped clarinets, mutilated trumpets and violins with snapped strings and warped fingerboards were presented as evidence; and the Chancellor decreed an end to these horrific and unmusical practices.
Henceforth, an Official Tuning Fork was to be produced, from which copies would be made and kept in various churches and universities, which would serve as the standard tone that instruments would be tuned to. The composers would simply have to make do with the instruments at hand. This new standard tone turned out to be a little flat of the Old C, which had for some decades been inflated to about D-sharp. Thus, July 12, 1566 was called the Great C to C-flat Transposition on account of the tuning note being altered from the Old C to Old C-flat.