From the Vatican Information Service:
The eighth General Congregation of the College of Cardinals has decided that the Conclave will begin on Tuesday, 12 March 2013. A “pro eligendo Romano Pontifice” Mass will be celebrated in St. Peter’s Basilica that morning. In the afternoon, the cardinals will enter into the Conclave.
Here is a nifty graphic with some interesting statistics on the last several conclaves. Fascinating stuff!