Your answer: Bonfire Night
Good guess, but the answer is actually "Cap Night." In the late 19th and early 20th centuries, many U-M students would walk from State St. to "Sleepy Hollow", an open plot of land where Couzens Hall and Alice Lloyd now exist, while singing Michigan songs. Every June, freshmen students would throw their caps into a bonfire to celebrate the end of the school year and symbolize their transition from first-year students to sophomores. It was lit.