The date was chosen to be symbolic of the second siege of Vienna. The event that, in classical historiography, marks the end of Muslim expansion in Europe and begins the shift of the dynamic in the favor of Christendom. They did those series of attacks so as to mark yet another turning of the tides they wished to set in motion. It was meant to be the 1st act in a decades long plan out of which (indirectly) Daesh was born. It was about striking at the capital of Western Christendom yet again as the Ottomans struck at Vienna.
