Why are days of the week proper nouns?

Is there any particular reason why days of the week are proper nouns?

Solution 1:

A proper noun names a specific member of a group: Janet, Asia, and Cadillac are proper nouns. Proper nouns are always capitalized.

This is what I find in English Grammar (ISBN 0-06-467109-7).

Asia is a member of the group of the continents, in the same way January is a member of the group of months, and Monday is a member of the group of weekdays.
As per definition of proper nouns, weekday names are proper nouns.

Solution 2:

They were formed from the names of old pagan gods (e.g. Friday—Freya's Day), so they are capitalized as proper nouns.

If you want to know where each originates from, see this page: The Seven-Day Week and the Meanings of the Names of the Days.