Yes, the word ‘Sunday’ is a proper noun.
A proper noun is a specific name used for an individual person, place, or organization, spelled with initial capital letters. In this case, ‘Sunday’ refers to a specific day of the week, distinguishing it from other days like Monday or Tuesday. Proper nouns are used to denote unique entities and are always capitalized to signify their significance. So, when you refer to ‘Sunday,’ you are naming a specific day and thus using a proper noun.