Complex sentence: It had been the theatre's policy to close during the summer while many city residents headed to the coast for their holidays, but this all changed in 1933 when Stewart Cruikshank decided to experiment with a summer show, something rather like seaside entertainment.
Split into simpler sentences: It had been the theatre's policy to be closed during the summer while the folks of the city all headed to the coast for their holidays. This all changed in 1933 when Stewart Cruikshank decided to experiment with a summer show, something rather like seaside entertainment.