I don't know why that day was chosen, but you can see if you can find a reason in this list of events that occurred in September 11.
My favorites are:
1609 - Henry Hudson lands on Manhattan island.
1776 - British-American peace conference on Staten Island fails to stop nascent American Revolution.
1777 - Battle of Brandywine
1922 - British Mandate of Palestine begins.
1941 - Ground broken for the construction of The Pentagon.
1998 - Independent counsel Kenneth Starr sends a report to the US Congress accusing President Bill Clinton of 11 possible impeachable offenses.
September 11