September 11 (2001), the day of terrorist attack on the U.S. by al-Qaeda, collapsing the World Trade Center twin towers in New York City; 9/11.
Note: this sign (pronounced as "9-11" is NOT the same for the emergency phone number 911 (pronounced as (9-1-1).
An ASL icon of September 11. Use the handshape "ILY". Sometimes phonological variation.