Senior Night is a well-established senior tradition, and perhaps one of the most successful. It's basically an excuse for seniors to start their drinking on Wednesday nights, instead of waiting until the customary Thursday like everyone else. Seniors gather at Havana Central for drinks, and there are usually drink specials or door prizes of some sort.
Officially, it is not a Columbia-organized event; it claims to be organized by the Senior Underground. Theories abound as to the identity of the Senior Underground, but remain as-of-yet unresolved. It's probably CCSC.