The phrase functions as a proper noun. It is the specific name of an official day of observance. In this construction, "Patriot Day" is the core name, and "United States" serves as a noun adjunct, a noun used to modify another noun, specifying the country associated with this particular observance.
This proper noun designates the American national day of remembrance observed annually on September 11th. Established by presidential proclamation, it honors the memory of the individuals killed in the terrorist attacks of September 11, 2001. The day is formally marked by specific protocols, including the direction for the U.S. flag to be flown at half-staff at all government buildings and establishments and a national moment of silence corresponding to the times of the attacks.
Recognizing the term as a proper noun is crucial for distinguishing it from descriptive phrases and from other holidays. For instance, it must be differentiated from "Patriots' Day," a separate regional holiday in some U.S. states commemorating early battles of the American Revolutionary War. The specific, capitalized title ensures legal and historical precision, anchoring its meaning to a single, solemn event in the nation's history.
