Russia became Soviet Russia after the October Revolution when the Bolsheviks led by V. I. Lenin seized power. However, it happened during the night of November 6 and 7, 1917. The anniversary of the October Revolution is commemorated on November 7.
How did this happen?
The calendar we now use is the Gregorian calendar, promulgated by Pope Gregory XIII in 1582. The calendar that was in use earlier was the Julian calendar, introduced by none other than Julius Caesar himself. However, there was a small error in the time of a year in the Julian calendar. The Gregorian calendar was a refinement to the Julian calendar involving a 0.002% correction in the length of the year.
However, in Orthodox Christian countries including Russia and Greece, the Julian calendar was in use. According to this system, the day the Gregorian calendar considered as November 7 was still October 25 in the Julian calendar.
Therefore, the revolution occurred in October according to the calendar used in Russia at the time.
The new Soviet government adopted the Gregorian calendar from the beginning of 1918. However, still the Christmas and other religious dates are celebrated in Russia according to the Julian calendar.