The discrepancy between the Julian calendar and the observations of astronomy had become unbearable by 1576, when Pope Gregory XIII launched a special commission to investigate how it might be reformed. The commission adopted the proposals of Luigi Giglio, who suggested suppressing 10 days, so as to realign the dates of the astronomical and canonical equinoxes. This new calendar was adopted in 1582, with the loss of 10 days in October. The Gregorian calendar was adopted the following year in Italy, Spain and Portugal. Great Britain adopted it in 1752, Russia in 1918 The Gregorian Calendar and the Lunario Novo of 1582

Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books