Yes, days of the week are considered proper nouns. This is because they refer to specific names of the days that are universally recognized and unique. For example, ‘Monday’, ‘Tuesday’, ‘Wednesday’, etc., are not just generic terms, but specific identifiers for particular days in a week.
In English, proper nouns are always capitalized, which is consistent with how we write the names of the days. This capitalization signifies their importance and helps differentiate them from common nouns. For instance, ‘day’ is a general term that can refer to any day of the week, but ‘Monday’ is a specific day, making it a proper noun. Similarly, we also capitalize months, holidays, and names of people, as they too are unique identifiers.
In summary, because days of the week name specific entities and are always capitalized, they qualify as proper nouns in the English language.