October 10, 1582
Due to the introduction of the Gregorian calendar, the day does not exist in Italy, Poland, Portugal, and Spain as the calendar shifts directly from October 4 to October 15.
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In October 1582, a significant moment in the history of timekeeping and calendrical reform occurred with the implementation of the Gregorian calendar. This change, driven by the need to correct inaccuracies in the Julian calendar, resulted in the removal of several days from the calendar, specifically in countries like Italy, Poland, Portugal, and Spain.
The Julian calendar, introduced by Julius Caesar in 46 BC, had become the standard throughout much of the Western world. However, it miscalculated the solar year by roughly 11 minutes per year. Over centuries, this slight error accumulated significantly, causing the calendar dates to drift with respect to the equinoxes and solstices, and misaligning with the timing of the Easter celebration, tied to the spring equinox.
In response to this drifting, Pope Gregory XIII introduced a new calendar. Issued by the papal bull Inter gravissimas on February 24, 1582, it proposed to realign the calendar with the equinoxes. Critically, the implementation required the omission of 10 days to realign the vernal equinox with March 21, the date it held at the time of the First Council of Nicaea in AD 325.
To correct this drift, the new calendar established that the day following Thursday, October 4, 1582, would be called Friday, October 15, 1582. Consequently, the dates from October 5 to October 14, 1582, were omitted entirely in these countries.
This reform initially took place only in Catholic countries such as Italy, Poland, Portugal, and Spain, as well as their colonies. Many Protestant and Orthodox countries were slower to adopt the new calendar, resulting in a patchwork of calendrical systems across Europe for centuries. England, for example, did not switch until 1752.
The Gregorian calendar ultimately improved the accuracy of timekeeping by simplifying leap year rules, which specified a leap year every four years, with exceptions for years divisible by 100 but not 400. Today, it is the most widely used civil calendar globally, serving as a testament to the necessity and durability of Gregorian reforms initiated over four centuries ago.
Source: en.wikipedia.org