[El Cid]

Some people are know speculating that Osama chose 9/11 because he believed that it was the true date of Christ’s birth. Seems more plausible than the meme that it was the American three digit emergency code.

September 11th (1565) was the date when the Knights Templar executed one of the greatest upsets in military history by breaking the siege of Malta and defeated the full weight of the Ottoman Empire (muslims).
Oddly enough the night of September 11th is also assosicated with the defeat of the Ottoman Muslims at Vienna - although September 12th is the date most commonly used.

Osama most likely picked September 11th simply as 'revenge' for the epoch defeat of the muslim Turks at Malta.

[El Cid]

If you are interested, Peter Hammond had a good talk about this siege (about 1 hr):

Peter Hammond - Siege of Malta

[Biggirl]

It was a Polish king who defeated the Muslims at the gates of Vienna.