Today is the celebration of Christmas in the Russian Orthodox Church. The Russian Orthodox Church uses the Julian calendar, which was introduced by Julius Caesar and was widespread in the Western Christian world until the Gregorian calendar was introduced in the 16th century. Most of the world adopted the Gregorian calendar, and the Julian calendar fell out of favor, but the Orthodox Church still follows it. These days, the Julian calendar is 13 days behind the Gregorian calendar — in 2100 that will change, and it will be 14 days behind — so for now, Russian Christmas is celebrated on January 7th.
via The Writer’s Almanac January 7, 2020