"Sesong" is not used for the four main seasons of the year in Norway, but rather for any period of time when something specific is happening, ripening, etc. The skiing season, the strawberry season, the tourist season.

We may talk of "sommersesongen" or "vintersesongen", but then it will be in relation to something like sports or trade, and not as names of the four main seasons like in the sentence we're discussing here.