Commem. is an abbreviation of Commemoration, and refers to the Commemoration balls at Oxford University.
These are formal balls held during the 9th week of Trinity (Summer) term. As Oxford terms are eight weeks long, this means they take place one week after the ending of the academic year.
Commemoration Week was set up to honour and remember the benefactors of the university.
Each major college will host a commem. ball once every three years. Smaller balls are held, often on an annual basis, at other colleges.