The term functions as a noun phrase, with "date" as the head noun. The specific date is September 11. This annual observance in the United States commemorates the victims of the terrorist attacks that occurred on September 11, 2001.
Officially designated as Patriot Day and National Day of Service and Remembrance, the observance was established by presidential proclamation in 2002 and later codified into U.S. law. Federal law directs that the flag of the United States be flown at half-staff on all government buildings and establishments. It also calls for a nationwide moment of silence to be observed at 8:46 a.m. Eastern Daylight Time, corresponding to the time the first plane struck the North Tower of the World Trade Center.
This observance should not be confused with Patriots' Day, a separate civic holiday celebrated on the third Monday in April in Massachusetts, Maine, and Wisconsin. Patriots' Day commemorates the Battles of Lexington and Concord, which marked the beginning of the American Revolutionary War in 1775.
