The Catholic page at http://www.catholic-hierarchy.org/event/c2013.html shows the conclave to elect the new pope will begin on March 16, 2013 although the same website at this page at http://www.catholic-hierarchy.org/conclave2013.html

says "Start of Conclave: unknown, Sistine Chapel
UDG #37 says 15 to 20 days after the vacancy, so between Friday night 15 March and Wednesday 20 March. Saturday 16 March or Monday 18 March seem likely options."