This was because the polar vortex was not in its normal position. No connection with global warming, it just happens from time to time. When the polar vortex is over Canada and USA they have the cold weather and the Atlantic winds keeps carrying humid and warm air over here.
This Xmas was not the warmest in record although it was warmer than most of the years. Finnish Polar Circle is only ~700km from the Atlantic and the same goes for rest of the Finland. Atlantic weather systems bring a lot of rain and mild weather here.
Now we have the "normal" Siberian high pressure system pushing cold air here, it is -15C in Helsinki, temperature in Lapland above Polar Circle is between -9 and -30C.
So, when you see extremely high variation from the average temperature it is definitely not the global warming, it is a weather system anomaly. Those are local, not global, and have happened since the beginning of the Earths atmosphere.