The Last Day of the Julian Calendar!

Tower of the Winds (Torre dei Venti), Vatican City

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October 4th 1582 was the last day of the Julian calendar. The next day the Gregorian calendar came into effect and ten days were omitted to make the adjustment from the old to the new. October 4th was thereby followed by October 15th!

The Gregorian calendar is named after Pope Gregory XIII (r. 1572-85). His funerary monument in St Peter's Basilica makes reference to his reformation of the calendar. 

The Torre dei Venti (Tower of Winds), in Vatican City, was expressly built to determine the extent of the inaccuracy of the Julian calendar. The eagle-eyed may spot a dragon atop the tower. (A dragon makes up the coat of arms of the Buoncompagni family, to which Pope Gregory XIII belonged.)