The reason many Russians and Ukrainians celebrate Christmas on the 7th of January is because after the Russian revolution the government switched to Gregorian calender but the Russian Orthodox church refused to make the switch and still uses the Julian calendar. The Julian Calendar is 13 days behind the Gregorian calendar. So while Russians or Ukrainians are in church, it's basically December 25th, as far as the church is concerned. Merry Christian Christmas to all the Russian/Ukrainian Orthodox folks and Merry Secular Christmas for all the Russian/Ukrainian non-theists.
"We swallow greedily any lie that flatters us, but we sip only little by little at a truth we find bitter." ~ Denis Diderot