Jahreszeit is used for the seasons of the year (spring, summer, ...) – those are Jahreszeiten – except in the context of holidays / vacations, there you can use something like Sommersaison / Wintersaison / Skisaison, for instance. If you talk about the arts (opera, theatre, fashion, etc.), you usually speak about about Saisons. In agriculture, both terms can be used (Spargelzeit, Spargelsaison, etc.).