Complex sentence: Its first "Ballet Evening" was presented in 1960; this is aside from its participation in operas and operettas, so the company prepares several evenings of ballet each winter season and performs at the Athens Festival each summer.
Split into simpler sentences: Its first "Ballet Evening" was presented in 1960. This is aside from its participation in operas and operettas, so the company prepares several evenings of ballet each winter season and performs at the Athens Festival each summer.