Every 'day' I have seen in the lessons for Spanish have the days of the weeks not capitalized. It still shows 'Friday' (English) with a capital 'F' but 'viernes' (Spanish) has no capital 'v'.
However, obviously, in Spanish you still capitalize names; so does that mean that Spanish simply has definition of 'proper nouns'? Or does Spanish not follow the 'capitalize proper nouns rule'? Or is this simply a computer error?
Spanish's capitalization rules are different that English's. You may have noticed that the months of the year are also not capitalized. I do not know the exact rules, but I know they are different. Hope this helps with your question!
Capitalization in Spanish: https://www.spanishdict.com/guide/capitalization-in-spanish