Complex sentence: On June 24,1870, the fifth anniversary of the society, a play called "The cousin from Saaremaa" ("Saaremaa Onupoeg") by Lydia Koidula was performed, marking the beginning of the Estonian national theatre and the theatre Vanemuine.
Split into simpler sentences: On June 24,1870, the fifth anniversary of the society, a play called "Saaremaa Cousin" ("Saaremaa Onupoeg" in Estonian) was peformed. This event marks the beginning of the Estonian national theatre and the theatre Vanemuine.