The freshmen get assigned to their upperclass houses the Thursday before spring break. The assignment is random (no Sorting Hat unfortunately) so, in order to get freshmen pumped about what is essentially a computer-generated slip of paper, the upperclassmen take over Harvard Yard going apesh*t with banners, music, and, yes, mascot costumes representing their houses.

(It's mad fun when you're involved. It's a bit unnerving when you're not.)