May 29 1453

On that day, a Tuesday, Constantinople, the Byzantine Empire's capital fell to the forces of the Ottoman Empire.

The event marked the end of the political independence of the millennium - old Byzantine Empire.

The fall of Constantinople accelerated the scholarly exodus of Byzantine Greeks which caused the influx of classical Greek studies into the European Renaissance.

In addition, it played a crucial role in Ottoman political stability and its subsequent expansion in the Eastern Mediterranean and the Balkans.

The date of the event is one of the frequently proposed events marking the end of the Middle Ages as a historical period.

(A nice, politically correct description, based on Wikipedia.)