During the winter it can be really full booked, so it is a very good idea if you are prepared and book a cottage or a hotel room. Sälen can offer you all kind of acommodations, but it can also be quite expensive. Be prepared that the price can be high for acommodation in Sälen.
If you know you'll go there, book an accommodation in advance.
The best seasons to visit this place is winter and summer. Even if in the summer time is not so busy, still you can have problems in finding a hotel room or a house to rent.
But don't worry, there are many enough and you just have to look more.