Oddly enough none of the answers thus far have answered the question correctly. Yes, 9/11 happened and is remembered annually on 9/11 but the exact date, which was the question, was on September 11, 2001.

See link for so much more information about this tragic event, which you only wouldn't know anything about if you lived in a cave until now.

http://en.wikipedia.org/wiki/9/11

On September 11

It happened on September 11, that's why it is called 9/11.

9/11 means September 11, in case you don't get it.

Oddly enough, on 9/11.

Every year.