The Collaborative International Dictionary of English v.0.48:

Class day \Class day\
   In American colleges and universities, a day of the
   commencement season on which the senior class celebrates the
   completion of its course by exercises conducted by the
   members, such as the reading of the class histories and poem,
   the delivery of the class oration, the planting of the class
   ivy, etc.
   [Webster 1913 Suppl.]