Is "Monday" a proper noun or a common noun?

I can understand why Monday is an abstract noun (it isn't something we can perceive with any of our 5 senses),

But is Monday considered a proper noun or a common noun?

Monday is considered to be a proper noun (in English the capitalization is a dead giveaway):

A proper noun or proper name is a noun representing a unique entity (such as London, Jupiter, John Hunter, or Toyota), as distinguished from a common noun, which represents a class of entities (or nonunique instance[s] of that class)—for example, city, planet, person or corporation)...

Which nouns are considered proper names depends on language. For example, names of days and months are considered proper names in English, but not in Spanish, French, Swedish or Finnish, where they are not capitalized.

Monday is a unique entity (representing the second day of the week), whereas the corresponding common noun, day, represents any day of the week.