Lapland in the spring
We are going on a 2-week trek in Finnish Lapland 27th may. As it turns out, there's more snow this year than on average. If the snow won't melt before our trip, we're planning on traveling at night when the temperature freezes the surface of the snow which makes it possible to travel on snow. During daytime it melts and won't carry you. At those latitudes, the sun won't set even at night so there won't be any problems with visibility. Added bonus is that potential mosquitoes will be hibernating when the temperature drops.
Lapland in the summer
The sun won't set so visibility is not an issue. It's also cooler than during the daytime.
Lapland in the fall
You're better off by traveling during the day as it's getting really dark and cold in the night time.
Lapland in the winter
Wind tends to calm down as the sun sets. Even in northern latitudes when the sun won't rise in the middle of the winter it is possible to travel in full moon if the sky is clear. Added bonus is the potential auroras which are just frickin awesome!!!