When the jihadists make a huge attack such as 9/11, etc., research the date. If you do, you'll usually see that something significant -- and usually bad for their side -- happened on that date, even if it was centuries ago. It's as if they see their choosing these dates as both advancing their cause and exacting revenge.
September 11 is no different. Vienna was theirs. It was in their grasp. That is, until Poland's King Jan (i.e., John) Sobieski came to the rescue. Christendom was saved. It also marks the the advent of drinking coffee in the West, but that's a different story.
Read more from Fr. Z in this short article here.