Complex sentence: Since its unveiling in 2006, his monument in Kraków has been vandalized three times: first on December 13 2011, (the anniversary of martial law in Poland) and then on February 11 2012 (the anniversary of Kukliński's death).
Split into simpler sentences: Since its unveiling in 2006, his monument in Kraków has been vandalized three times. First on December 13,2011 (the anniversary of martial law in Poland) and then on February 11,2012 (the anniversary of Kukliński's death).