The calendar change that the Soviet government implemented was decreed in late January 1918: January 31, 1918 was to be followed by February 14, 1918.

The Russian Orthodox Church refused to adopt the Gregorian Calendar, which is why it still celebrates Christmas on January 7 - the Soviet government had officially announced separation of church and state previous to the calendar reform, so in theory the state calendar change didn't impact a religious calendar anyway.

So that's the "simple" answer. But it's probably worth noting that the Soviet government actively discouraged any celebrations relating to Christmas between their taking of power and 1936, and that from that date until the present, most public celebrations were secularized and moved to New Years Day. So January 7 was long only a religious holiday separated from the public holiday.