Grammatically, the phrase is an interrogative clause, or a question. It is not a single part of speech but a complete sentence structured to elicit specific information. Its core components are an interrogative pronoun ("qu," meaning "what"), a noun ("da," meaning "day"), and a verb ("fue," the past tense form of "ser," meaning "was"). The date "11 sep 2001" functions as an adverbial phrase of time, specifying the context for the question. The literal English translation is, "What day was September 11, 2001?"
The direct answer to the question is that September 11, 2001, was a Tuesday. The use of the verb "fue" (from "ser") is significant in Spanish, as "ser" is used to describe permanent or inherent characteristics and identities. The day of the week for a specific date is considered a fixed, identifying characteristic, making "ser" the appropriate verb choice over "estar," which is used for temporary states or locations.
In application, the phrase serves as a precise query to retrieve a specific piece of calendrical data. It seeks to associate the given numerical date with its corresponding weekday name. This structure represents a common and grammatically correct method in Spanish for inquiring about the day of the week for any past date.
