The correct grammatical form is "September 11th." The suffix "-st" is used for ordinal numbers ending in one (e.g., 1st, 21st, 31st), with the notable exception of eleven, which uses "-th." As a keyword or term within a text, "September 11th" primarily functions as a proper noun because it designates a specific, unique calendar date.
While its primary classification is a proper noun, the term can also function as an adjective (specifically, a noun adjunct) when it modifies another noun. The part of speech is determined by its role within a sentence. For instance, in the sentence, "The world changed after September 11th," the term is a noun, acting as the object of the preposition "after." Conversely, in the phrase, "the September 11th attacks," the term modifies the noun "attacks," thereby functioning as an adjective.
For the purpose of your article, determining the main point requires analyzing the term's context. If the article focuses on the date itself as a subject, it is a proper noun. If the term is consistently used to describe related events, commissions, or memorials, its adjectival function is more prominent. This distinction is crucial for maintaining grammatical precision and clarity in your writing.
