There has been a great deal of speculation about this. Many believe
that the date may have been chosen because September 11 was a very
important date in Islamic history. The Ottomans (Muslim Turks)
suffered defeat in the Battle of Zenta on September 11, 1697. Prince
Eugene of Savoy and his troops killed 20,000 Ottomans, seized the
Ottoman treasury (with its artillery and provisions) and took captive
ten of the Sultan's wives. After the battle, the treaty of Karlowitz
was signed, in which the Ottomans ceded Croatia, Hungary,
Transylvania, and Slavonia to Austria.
"The last effort they [Muslims] made to destroy Christendom was
contemporary with the end of the reign of Charles II in England and of
his brother James and of the usurper William III. It failed during the
last years of the seventeenth century, only just over two hundred
years ago. Vienna, as we saw, was almost taken and only saved by the
Christian army under the command of the King of Poland on a date that
ought to be among the most famous in history-_September 11, 1683. 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 Zenta in 1697 and the capture of Belgrade that the tide
really turned_and by that time we were at the end of the seventeenth
From "The Great and Enduring Heresy of Mohammed," by Hilaire Belloc
(written in 1936).
EWTN: The Great Heresies
Yet another historically significant occurrence of this date is
related to the Battle of Vienna:
"I've also frequently heard it suggested that the timing of the recent
attacks was linked somehow to the signing of the Camp David agreement
in September 1978. I thought this sounded a plausible reason for the
death-squads circling that date in their diaries - until I discovered
that the agreement was in fact signed on September 17. Fanatics don't
make mistakes like that.
I now think I can provide a more persuasive explanation, however. 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. 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."
Guardian Unlimited: Why the suicide killers chose September 11
"Did the terrorists of al-Qaida know the significance of the date they
picked to launch their war against the West? September 11, in 1683,
was the date Muslim armies were turned back from Vienna."
The Oak Tree: Reflections in a Time of War
"A little over a century after the Battle of Lepanto, another
momentous battle occurred between Catholic and Mohammedan Turkish
forces. Again, the stakes were high -- in this case, the city of
Vienna and a temptation to the Turkish forces to press deeper and
deeper into Europe. 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. He sent to Pope Innocent XI a letter that proclaimed
Veni, vidi, Deus vicit [I came, I saw, God conquered], a play on Veni,
vidi, vici [I came, I saw, I conquered], Julius Caesar's classic
summary of his swift victory at the Battle of Zela in 47 B.C. over
King Pharnaces in the Pontic campaign.
Note that the disasters of September 11, 2001, occurred on the same
day that John Sobieski defeated the Mohammedans in 1683 in Vienna.
Catholic Apologetics Information: Real Islam vs Imaginary Islam
It should be noted that some sources give the date of the Battle of
Vienna as September 12 or September 13, 1683. The nature of battles
sometimes makes it difficult to set a certain date as a
Another interesting theory is that the terrorists chose that date
because of the date's significance to Palestine:
"The Oaths of office for the High Commissioner and Commander in Chief
for Palestine were administered at Jerusalem on September 11, 1922. So
that is 9-11-22 when the British Mandate for Palestine was officially
begun with the swearing in oath.
In the video, bin Laden said this:
When the sword reached America after 80 years...
1922 to 2001 is 79 years, but perhaps that is the 80 years he refers to."
Free Republic: Why 911 - September 11 1922 British mandate of Palestine begins
"The Economist Oct. 15, 2001 edition about the attack on the World
Trade Center and Pentagon, noting 'the day a British mandate came into
force in Palestine, over the heads of unyielding Arab opposition,'
quoted from a dispatch from Jerusalem to London's The Times of
1922.'"The Arabs declared a day of mourning throughout the city and
the shops were closed as a protest against today's formal proclamation
of the Mandate, but no Jews were molested.' - The day was September
Palestine Chronicle: Who Else Remembers September 11?
Another theory, related to the United Nations:
"In 1981, the General Assembly declared 21 September the International
Day of Peace, which should be dedicated to 'strengthening the ideals
of peace both within and among all nations and peoples.'
In 1998 the assembly changed the date of the International Day of
Peace so that in 2001 it would be 11 September.
It seems likely this was why that date was chosen for the terrorist
acts committed in New York."
BBC: Local remembrance events for September 11
Here you'll find a speculation that the date was not chosen by the
terrorists, but by the United States Government:
The People's Investigation of 9-11: The U.S. Government, Not the
Hijackers, 'Chose' the Date of the 9-11 Attacks
In case you're curious about other historical events that occurred on
this date, there's a long list here:
Brainy History: September 11 in History
Google Web Search: "significance of the date" "september 11"
I hope this information is useful! If anything is unclear, or if a
link doesn't work for you, please request clarification.