Today is the 554th anniversary of the Fall of the Byzantine Empire. To be more precise, on this day in 1453, the defenses of Constantinople collapsed before the Turkish army under Mehmed II. Strictly speaking, a remnant of the old empire, the Empire of Trebizond, which had formed in the aftermath of the Fourth Crusade, still remained to carry on the succession of the Roman Empire, as well as a few fragments elsewhere; but they had less than a decade to survive.
As eyewitness Nicolo Barbaro put it, "On the twenty-ninth of May, the last day of the siege, our Lord God decided, to the sorrow of the Greeks, that He was willing for the city to fall on this day into the hands of Mahomet Bey the Turk son of Murat."
The Feast of St. Constantine XI Palaeologus the Ethnomartyr, last of the Byzantine Emperors, is celebrated on this day in some parts of the world.