Complex sentence: The first season after the end of the Civil War opened on 20 August 1921 and, during World War II, during the period when the Leningrad State Academic Theatre of Opera and Ballet evacuated its personel to the city, performances were given in the Perm Theatre.
Split into simpler sentences: The first season after the end of the Civil War opened on 20 August 1921. During World War II, during the period when the Leningrad State Academic Theatre of Opera and Ballet evacuated its personel to the city, performances were given in the Perm theatre.