Did the terrorists of al-Qaida know the significance of the date they picked to launch their war against the West?
There has been a great deal of speculation about this, but yes I think that they did.
It was on September 11,1683 that the conquering armies of Islam were met,held, and thrown back at the gates of Vienna. Now this, of course, is not a date that has only obscure or sectarian significance. In July 1683 the Grand Vizier Kara Mustapha led his Turkish troops to Vienna and laid siege against the city. On September 11,1683 a decisive battle was fought before the city walls. After an initial setback, Poland's great military leader, and later king, John Sobieski, with his forces stormed the enemy camp and routed its army. It can rightly, if tritely, be called a hinge-event in human history. The Ottoman Empire never recovered from the defeat; from then on it was more likely that Christian or western powers would dominate the Muslim world than the other way around... in the Islamic world, and especially among the extremists, it is remembered as a humiliation in itself and a prelude to later ones. But the peril remained, Islam was still immensely powerful within a few marches of Austria and it was not until the great victory of Prince Eugene at Battle of Zenta on September 11,1697 and the capture of Belgrade that the tide for the Ottomans really turned.
There may be operational reasons why this date was chosen by al-Qaida, but I'm sure that it was chosen because of its importance of the Battle of Vienna.