The Sälen season runs from late November to early May, but the heart of it is February and March — long, light days, reliable snow, and the Vasaloppet weekend on the first Sunday of March. Book early for the school break weeks (Stockholm's sportlov in week 9 is the busiest).
Summer is short and quiet. July is the most reliable month for hiking in Fulufjället; bring rain gear and a serious mosquito net. The autumn colours in September can be spectacular but the days are already short.
Avoid early December if you want guaranteed snow — the slopes open as soon as they can be machine-covered, but conditions can be patchy until Christmas.