I've never really understood the concept of "casual Friday." To me, Fridays are the best day of the week. They're the start of the weekend, the gateway to a little bit of free time and extra sleep. Plus, I don't have class 'til 2pm on Fridays. Friday is my favorite day to dress up.
Mondays are a whole different story.
Everyone knows Mondays are awful. They are a terrible way to spend 1/7th of life, which is why I often observe "casual Monday." Easing into the week in a super soft basic tee and comfy jeans makes Monday a little bit better, and a better Monday means a better week.