Complex sentence: The 30th Salon des artistes rouennais went ahead as scheduled, with its opening on 19 May 1940; however, due to the gravity of the situation, it was closed two days later.
Split into simpler sentences: The 30th Salon des Artistes Rouennais went ahead as scheduled, with its opening on 19 May 1940. Notwithstanding, due to the gravity of the situation, the Salon was closed two days later.