In 1582, in Italy and Spain, this day became the first day of the Gregorian Calendar after it was adopted by Pope Gregory XIII.

CNN: Tuesday,

The so-called Meridian Hall, created to verify the accuracy of the calendar Pope Gregory XIII promulgated in 1582, is in the Vatican Secret Archives, which hold some 10m documents stored by the papacy over the past 1, 200 years.

ECONOMIST: The Vatican turns a page��slowly

Ask Julius Cesar and Gregory XIII!

UNESCO: International Women��s Day: why the 8 of March?

Before the Revolution, Russia had not yet adopted the Gregorian calendar, introduced by Pope Gregory XIII in 1582 to mitigate the errors of the Julian calendar, which owes its name to the Roman emperor who had chosen it 46 years before the birth of Jesus Christ.

UNESCO: International Women��s Day: why the 8 of March?

There were three rival claimants to the papal throne at that time - the Roman Pope Gregory XII, the Avignon Pope Benedict XIII, and the Antipope John XXIII.

BBC: How does a Pope resign from office?