Using the Julian calendar
Some jurisdictions of the Eastern Orthodox Church, including those of Russia, Georgia, Ukraine, Macedonia, Montenegro, Serbia, and Jerusalem, mark feasts using the older Julian calendar. As of 2015, there is a difference of 13 days between the Julian calendar and the modern Gregorian calendar, which is used internationally for most secular purposes. As a result, December 25 on the Julian calendar currently corresponds to January 7 on the calendar used by most governments and people in everyday life. Therefore, the aforementioned Orthodox Christians mark December 25 (and thus Christmas) on the day that is internationally considered to be January 7.