Russia is very multinational country. The biggest ethnicity in the country - Russians are Orthodox. On the pictures above you see several Orthodox churches in Moscow.
Orthodox church celebrates Christmas 7th January (not 25 December).
7th of January is a day off in Russia.
In general, if you plan to come to Russia for business needs never plan your trip between 29 December - 13 January. 1, 2 and 7 of January are always days off plus Saturdays and Sundays. Basically the first half of January is celebration time.