In December 1917, a few courageous Finns decided to relieve Russia of the burden of governing Finland. They immediately left the capital-city-to-be, Helsinki, to go and tell the Russian czar about their decision. Because of the long journey (they accidentally took a wrong turn and ended up traveling around the world before reaching their destination), it took them almost two months to get to St. Petersburg. The Finns were, frankly, a bit surprised to hear how much had happened during this time, the least of which was not that there was no czar any longer.
The Finns were, however, adamant that Finland had actually declared independence in December of 1917, and not in February of 1918. A meeting was organised with the new government of Russia to discuss the difference in dates. There was never any doubt of Finnish sovereignty; Russia was glad to see Finland go. A lot of compromises were proposed, for example that the day of declaration would be February 6, 1918, not December 6, 1917 (it was actually February 4, 1918 at that time), but this did not suit the Finns as they feared it would be confused in the future with the new election month, Febtember.
After hours and hours of tiring negotiations, a solution that satisfied everyone present, was found by no other than Vladimir Ilyitsh Putin, then leader of Russia, himself. The day Finland declared independence would be called December 66, 1917.