The October Revolution actually started on November 7th, 1917.
The discrepancy arises because
Russia did not adopt the Gregorian calendar until about five centuries after
it was instituted by Pope Gregory XIII in 1582.
The Gregorian calendar corrects inaccuracies in the Julian calendar which accounts
for the differences in dates.
In terms of the Old Style Russian date, the Revolution started on October 25, 1917.
If you want to learn more about the origins of the current calendar, read the interesting book
Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year
by David Ewing Duncan.