The keyword phrase specifies a date, "11 Sep," and inquires about the corresponding day of the week. In its most historically significant context, September 11, 2001, was a Tuesday. Grammatically, the core component of the query is the date "11 Sep," which functions as a proper noun phrase. The analysis seeks to identify another nounthe specific name of the day of the week.
The term "11 sep which day" is an interrogative phrase composed of several parts of speech. "11" is a cardinal number functioning as an adjectival modifier for the noun "Sep" (an abbreviation for the proper noun September). The combination "11 Sep" forms a noun phrase that identifies a specific calendar date. "Which" is an interrogative determiner modifying the common noun "day." The day of the week for any given calendar date changes annually. Due to the 365-day common year, a date advances by one day of the week each year, plus an additional day following a leap day (February 29). For example, September 11 was a Monday in 2023 and will be a Wednesday in 2024.
The practical application of this analysis is distinguishing between a fixed date and its variable attribute. The date "September 11" is a fixed point in the Gregorian calendar, functioning as a noun. The associated day of the week (e.g., Tuesday, Wednesday) is also a noun, but its value is contingent upon the year. While the date can fall on any day, the historical context of the year 2001 has permanently linked "September 11" in the public consciousness with "Tuesday."
