The Russian Orthodox Church uses the abandoned Julian calendar. January 7th on the Julian calendar corresponds to December 25th on the Gregorian calendar which we all use.
Not just the Russian Orthodox celebrate Christmas on January 7th (Three Kings Day) - all Orthodox churches do.
“The Russian Orthodox Church uses the abandoned Julian calendar. January 7th on the Julian calendar corresponds to December 25th on the Gregorian calendar which we all use.”
Yes, it does use the antiquated Julian calendar — but only for movable religious feast days: It follows the Gregorian calendar for everything else.
Pretty much the whole world — with the exception of a handful of Moslem countries (Iran, Afghanistan, Saudi Arabia) and Ethiopia — uses the Gregorian calendar for civil and secular dates (as does Russia).
And the Julian calendar’s January 7 only corresponds to the Gregorian December 25th at the present time; it will change with the calculation of leap year (which is one of the big differences between the Gregorian calendar and the Julian calendar).