The phrase "11 septiembre de" is an incomplete noun phrase (in Spanish, sintagma nominal or frase nominal). Its primary function is to act as a noun within a sentence. The core of this construction is the noun "septiembre" (September), which is modified by the numeral "11." The preposition "de" (of) is included to connect this date to a specific year, which is absent in the provided fragment.
In Spanish, the standard grammatical structure for a full date is `[numeral] de [month] de [year]`, for example, "11 de septiembre de 2001." The provided phrase is a fragment of this structure. The head of the phrase is the noun "septiembre." The number "11" functions as a determiner, specifying the day. The final preposition "de" signals the beginning of a prepositional phrase that would modify the date, typically by adding the year. Without the year, the phrase is grammatically incomplete but its function as a nominal unit remains.
For the purpose of an article, this phrase should be treated as a proper noun that refers to a specific, historically significant date and the events associated with it. It can function as the subject of a sentence (e.g., "[The events of] 11 September changed global politics"), the direct object of a verb (e.g., "The world remembers 11 September"), or the object of another preposition (e.g., "in the years after 11 September"). Its classification as a noun phrase is the key to its correct grammatical use and analysis.
