The official start date of the 2013 papal conclave is March 12, 2013. The conclave will begin with a “pro eligendo” Mass, offered on behalf of the papal election, in St. Peter’s Basilica, followed by the first session of voting in the Sistine Chapel in the afternoon. Cardinal Prosper Grech, a Maltese expert in the early church fathers, will deliver a meditation at the beginning of the conclave. Read more at Catholic News Service.
Pope Benedict XVI’s resignation became effective on February 28, 2013. As Pope Benedict stated in his announcement to the cardinals:
Well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of St. Peter, entrusted to me by the cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of St. Peter, will be vacant and a conclave to elect the new supreme pontiff will have to be convoked by those whose competence it is.
Under the rules, the starting date of a conclave is between 15 and 20 days after the See becomes vacant, or in this case, March 16, 2013 or after. In a Motu Proprio issued in February 2013, just days before his resignation became effective, Pope Benedict XVI gave permission to the cardinals to make an exception and start the conclave earlier, provided that all of the cardinal electors have arrived in Rome.
The college of cardinals was waiting for the arrival of the final elector in Rome before choosing a date for the conclave.
For further discussion, see our related question When does the voting begin? At what time please?
and the post When will the conclave start? on the blog Canon Law Made Easy