Pope Gregory XIII, who gave his name to the calendar most of the world now uses.Image: Attributed to Bartolomeo Passarotti · Public domainThe day after the fourth of October was the fifteenth
In the Catholic countries of Europe, 15 October 1582 was the day that followed 4 October 1582. Ten dates in between simply did not exist. The papal bull Inter gravissimas, issued in February that year, had ordered them struck out to drag the spring equinox back to 21 March, where the Church’s calculation of Easter assumed it would be. The problem was arithmetic. Julius Caesar’s calendar made the year about eleven minutes too long, which sounds harmless and adds up to roughly a day every 314 years. Over sixteen centuries it had pushed the equinox ten days out of place. The new scheme kept leap years but dropped three of them every four hundred years, and October was chosen because nothing important in the Church’s year would be lost. Protestant and Orthodox states refused to take instruction from Rome, so Europe kept two calendars for centuries; Britain and its colonies did not change until 1752.
Source: Encyclopaedia Britannica: Gregorian calendar ↗
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