Complex sentence: On 21 September 1833, the curtain in the now newly-built theater (today's Staatstheater Mainz) on Gutenberg Square rose for the first time for Carl Maria von Webers 9th "Jubilee Overture" and Mozart's "La clemenza di Tito".
Split into simpler sentences: On 21 September 1833 the first curtain in the now newly-built theater (today's Staatstheater Mainz) on Gutenberg Square rose. It was played Carl Maria von Webers 9th "Jubilee Overture" and Mozart's "La clemenza di Tito".