If you hate big mosquitos yet don't want to go in the midle of winter, spring and fall are best, I think. I have never been in Sweden, but it applies to the whole boreal forest area, I feel. I have done some planning for a trip to Sweden.
In the middle of winter there is the risk of heavy snowfall. The summer brings lot's of mosquitos.
Februari, March are good I think. In Februari there is some risk of snow still, I reckon... Fall brings a bit more rain then spring of course..., so make your choice
What kind of trip is it you are going to make? Bushcraft, Hiking, Camping? What part of Lapland (North or South?)