Every student who has ever attended Mercersburg can claim being a member of either Irving or Marshall. These two societies participate annually in a fun-loving but fierce mid-winter competition connected to a long and serious tradition of debating at the school.

In fact, the societies are the school's oldest organizations—older than the school itself. Precedent for societies began in 1835, when the original students of Marshall College maintained two societies, the Diagnothian and Goethean literary societies. In 1873, they were renamed The Washington Irving Literary Society and the John Marshall Literary Society. The first debate occurred in 1894, under Mercersburg's founding Headmaster William Mann Irvine on Washington's Birthday. This began the tradition of midwinter weekend competitions in February.

Although it has evolved, the competition continues today as a spirit-filled week of rivalry in a variety of events, such as pool, chess, swimming, wrestling, and basketball. The week culminates in Declamation, where individuals present practiced monologues as representatives of their societies. Afterwards, students attend a dance, where the results of the speaking contest and the winning society are announced.