Berating a professor like a group of mobs seems a bit extreme, but I don’t see how having a no-white day is a problem. An all-minority event could be empowering to said groups. If you disagree with this so much, do you also disagree with all-boy or all-girl high schools? They still exist like literally everywhere.
Decades ago every day in America (and beyond) was a no-black day, or a no-woman day, or a no-gay day. Now white people can have a taste of what it was like to be black, sounds rather educational to me. If I were white and I went to that school, I would respect this and not be on campus for 24 hours.