Today is Patriot Day, a day designated in 2001 to remember the surprise attacks on the United States by Arab terrorists. Here is the background story from Encyclopedia Britannica:
Patriot Day, holiday observed in the United States on September 11 to commemorate the lives of those who died in the 2001 terrorist attacks on the World Trade Center in New York City and the Pentagon in Virginia and those who perished when the hijacked United Airlines Flight 93 crashed in Pennsylvania. The holiday also recognizes those who died attempting to rescue people trapped by the attacks.
By a joint resolution of the U.S. Congress on Dec. 18, 2001, September 11 was designated as Patriot Day. The resolution calls for the president of the United States to issue a yearly proclamation requesting that all U.S. flags be flown at half-staff.
Americans are asked to honour the dead with a moment of silence beginning at 8:46 AM, Eastern Standard Time, the time that the first airplane struck the World Trade Center, and to respect the ceremonies of remembrance when they are conducted.
Prior to passage of the resolution, several other names had been proposed for the day, such as the National Day of Remembrance and the National Day of Prayer and Remembrance.