When did countries change from Julian to Gregorian calendars?
The papal bull of February 1582 decreed that 10 days should be dropped from October 1582 so that 15 October should follow immediately after 4 October, and from then on the reformed calendar should be used. This was observed in Italy, Poland, Portugal, and Spain. Other Catholic countries followed shortly after, but Protestant countries were reluctant to change, and the Greek orthodox countries didn’t change until the start of the 1900s. Changes in the 1500s required 10 days to be dropped. Changes in the 1600s required 10 days to be dropped. Changes in the 1700s required 11 days to be dropped. Changes in the 1800s required 12 days to be dropped. Changes in the 1900s required 13 days to be dropped. For example, when Soviet Russia undertook its calendar reform in February 1918, they moved from the Julian calendar to the Gregorian. This move resulted in a loss of 13 days, so that February 1, 1918, became February 14. The following list contains the dates for changes in a number of countries
