The keyword phrase is a grammatical fragment. The main point, and the grammatical subject of any sentence built from this fragment, is the noun phrase "11 sep". This proper noun designates a specific date, September 11th, and serves as the central topic to be defined or described.
The remaining words in the fragment modify this central noun. "Celebrated" is a past participle, functioning here within a reduced passive clause (a shorthand for "[which] is celebrated"). It describes an action performed upon the subject ("11 sep"). The word "as" is a preposition that introduces the specific name, title, or capacity in which the date is commemorated (e.g., "as Patriot Day"). The entire structure serves to link the noun ("11 sep") to a new identity or designation.
For the purpose of structuring an article, the crucial part of speech to focus on is the noun. The article's core subject is the date itself. The phrase "celebrated as" provides the definitional context, explaining what the noun has become or how it is officially recognized. Therefore, the analysis is fundamentally about the noun and its reclassification.
