The term functions as a noun phrase. Its core grammatical element, or head, is the noun "movie." The preceding words, "patriots day marathon," act collectively as a compound modifier specifying the type of movie.
In this construction, "movie" is the subject being described. The word "marathon," a noun in its own right, serves as a noun adjunct (or attributive noun), functioning adjectivally to classify the film's topic. Similarly, the proper noun "Patriots Day" modifies "marathon," specifying the particular event. This creates a chain of modification where each preceding word further specifies the noun that follows it, culminating in a highly descriptive label for the final noun, "movie."
This grammatical structure allows for precise classification. The phrase does not refer to any movie about a marathon, nor any movie about the holiday itself, but specifically to a cinematic work centered on the marathon that occurs on that particular day. The entire phrase operates as a single conceptual unit to identify a specific film, such as the 2016 production titled "Patriots Day."
