On the morning of September 11, 2001, America was viciously attacked by terrorists. According to Britannica, there were ultimately close to 3,000 Americans who lost their lives that day. This act of terrorism forever changed how the world looked at security and how Americans looked at one another.
On December 18, 2001, a joint resolution appointed September 11 as "Patriot Day," ordering the flag to be put at half staff across the USA in honor of all those who suffered and paid the ultimate sacrifice on September 11, 2001.
More information on the official proclamation can be found here: Proclamation.