January 6 is the day traditionally known as Old Christmas. It’s a relic of the Julian Calendar, developed 2,000 years ago. In the late 1500s, Pope Gregory XIII changed the calendar to match the solar cycle more closely. To do so, the Julian Calendar had to be reduced from 376 to 365 days, eliminating 11 full days. Some countries, though, resisted the change and kept the old Julian Calendar. It took nearly 200 years for England and Scotland to come around. Both countries adopted the Gregorian Calendar in 1752.