The conclave to elect the successor of Pope Benedict XVI will start on March 12, 2013; according to a release by the United States Council of Catholic Bishops.
Details are as follows, summarized on the USCCB facebook page. "The Conclave will begin on Tuesday, 12 March 2013. A “pro eligendo Romano Pontifice” Mass will be celebrated in St. Peter’s Basilica in the morning. In the afternoon the cardinals will enter into the Conclave."
The 115 cardinals who will be voting have made their way to Rome, with the last cardinal arriving Thursday. They then met to decide the date to start the conclave on. Due to the advanced notice by Pope Benedict XVI that he was resigning, the traditional fifteen day wait before starting the conclave has been waived.
The conclave is set to start nineteen days before Easter. As a conclave hasn't lasted over four days in almost 200 years, it is most likely that the Catholic church will have a new leader and the cardinals will be back at their cities in time for Holy Week and Easter.