The keyword phrase "september 11th special day" functions grammatically as a noun phrase. The core element, or head, of the phrase is the noun "day," which is modified by the preceding words.
In this construction, "day" is the central noun. It is described by two modifiers. The first is "special," a standard adjective that qualifies the type of day. The second is "september 11th," a proper noun (a specific date) functioning as an adjectival modifier. It specifies precisely which "special day" is the subject, similar to how "brick" modifies "house" in the phrase "brick house." Therefore, the entire unit acts as a single noun in a sentence.
As a noun phrase, this keyword term can serve as the subject, object, or complement within a sentence. This grammatical function allows it to be the central topic around which an article is built. For example, it can be the subject ("September 11th special day is a time for reflection.") or the object ("The nation observes September 11th special day.").
