The reason for changing the calendar was that people realised that there is a mistake in the Julian calendar, that it adds too many leap years. This meant that the date of Easter was being calculated wrongly, because it depends on March 21 being the day before the equinox. So they designed a new calendar that corrected this error. The new rule was that a leap year would happen every four years (as before) "but not" if the year ended in 00 "unless" it could be divided evenly by 400. (So 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not.) Pope Gregory XIII had declared that this new calendar should be used from 1582 onwards. But only Roman Catholic countries accepted this ruling: Protestant and Eastern Orthodox countries refused to have the Pope tell them what to do. So even in Europe, the change happened at different times.