Well, depends.
Summer is short, so if you don't like cold, try July and August. You'll find that it's hard to avoid rains, but draughts occur, you just never know when! It's never too hot in Scandinavia.
Watch out for mosquitos in Northern Sweden and Finland in the summertime
Norway is a terrific place to go in the wintertime if you like skiing, one of the best in the world.
Museums are plenty and open all year, so there's no seasons...
Weather is normally bad, grey and rainy in October, November, March and April.
But come anytime! Your welcome