Coldest I've ever been on a bike was an hour or two of freezing sea fog on the way back from Nordkapp in September. Maybe it wouldn't have been so bad if I'd known it was coming, but it just seemed to appear out of nowhere on an otherwise perfectly nice day (i.e., 5C/40F and sunny). There were also fog-colored reindeer wandering across the road at random, and with visibility very poor and me shivering and my visor icing I was altogether miserable for a bit.
And then it was over, and I had sunny weather all the way through Finland until I hit rain in Helsinki. A wonderful area, not to be missed; just be sure to dress right and be prepared for anything.