Use Instrumental to express 'in' a particular season.
Ћсенью/Лљтом чђсто шёл дћждь. It often rained in the fall/summer.
Зимћй/Веснћй мІ чђсто хћдим в теђтр. We often go to the theater in winter/spring.
But the word for 'season' itself (врљмя гћда 'time of year') is used in the Accusative with the preposition в:
В іто врљмя гћда дћждь не идёт. It doesn't rain in/during that season.
У нђс во всљ временђ гћда идџт сќльные дождќ. In our country we have hard rains in all seasons.