Marijuana enthusiasts have long visited the University of Colorado at Boulder for a pot-smoking celebration on April 20. The university announced Monday that it would be shutting its campus to outsiders on April 20 this year -- part of a move to discourage the event. Students and employees will need identification cards to get on campus. “The gathering disrupts teaching and research right in the heart of the campus,” said CU-Boulder Chancellor Philip P. DiStefano. “The size of the crowd has become unmanageable, and limits our faculty, staff and students from getting to class, entering buildings and doing their basic work. It needs to end.” A legal challenge to the campus restrictions is possible. Officials from the American Civil Liberties Union told The Denver Post that the annual event is not just a party, but a political protest against drug laws, and that public universities cannot bar peaceful protests (or peaceful protesters) from campuses.