Back in the year 1582, pope Gregory XIII found that the inaccuracies of the Roman calendar were getting out of hand. Although a leap year had kept aberrations on the calendar down, it now transpired that the earth was spinning 10 days ahead of schedule. His religion being Christian rather than Muslim (which states that when Mohammed can't come to the mountain, the mountain comes to Mohammed), Gregory decided to take a drastic step. He commanded that the calendar be moved forward 10 days. During the last months of 1582 and the early ones of 1583, this was implemented across Europe. Another change was that leap years in start years of new centuries (e.g. 1800, 1900, 2000) would only occur if the year number was divisible by 400.
Protestants were not always eager to follow a papal decree. In Scotland, the Old New Year was kept until fairly recently in some quarters. This led to Christmas being celebrated on January 6th (in fact the Epiphany). However, it should be pointed out that formally, the corrections of the Gregorian calendar were accepted across the world by 1923.