In 1582 Pope Gregory XIII issued the bull Inter Gravissimas with which he introduced the modification of the calendar that will take his name, the Gregorian Calendar.
Until then, the Julian Calendar had been in force, introduced by Giulio Cesare in 46 BC.
The modification was based on the calculations made by the Polish astronomer and mathematician Niccolò Copernico who in 1543 in his De Revolutionibus orbium coelestium had recalculated the subdivision of the solar year. From that moment each year would have had 365 days, or 366 in the case of a leap year, grouped into twelve months with a duration ranging from 28 to 31 days.
“We prescribe and command as regards the month of October of the year 1582 that ten days be removed, from the third day before the nones, to the day before the ides included“
The Gregorian Calendar was then introduced on October 4th 1582 and, to realign it to the Julian Calendar, it was necessary to eliminate ten days by passing directly to October 15.
And therefore from 5 to 14 October no one was born, no one died. Nothing happened …
Why start in October? Simply because it was the month with the fewest religious holidays and caused the least disturbance for the Church.
Florence and Tuscany also rebelled against the Pope (it was nothing new …) and was forced to adopt the Gregorian Calendar starting from January 1st 1750, 168 years behind all the others.
Francesco di Lorena, Grand Duke of Tuscany, established that the Grand Duchy also conformed to the many European countries already aligned and that from that moment the New Year was celebrated no longer on March 25th – as was tradition in Florence – but on January 1st.
Many states of “Protestant” religion as well as China and Turkey joined the reform only at the beginning of the twentieth century.