Today, September 4, is one of the two most popular choices. If you think of the Roman Empire with its epicenter as the city of Rome, then on this day in 476 is when the last Western Roman emperor, Romulus Augustulus, was tossed from the city by the last of a wave of eastern invaders. This one, Odeacer, couldn’t even be bothered executing the guy. That’s how big a threat Romulus and the Roman Empire was at the time.
However, Constantine shifted the seat of the Roman Empire east, and set up “New Rome” on the narrows between Europe and Asia. The city became known as Constantinople, after him, and the Roman Empire lasted for nearly another thousand years – until May 1453, when the Ottoman Turks finally breached the city walls.