Question: The Papal conclave was convened to elect a pope following a resignation was first announced on the morning of what date?

Context paragraphs:
Papal conclave, 1455: The papal conclave from April 4-8, 1455 elected Alfons Borja Pope Callixtus III following the death of Pope Nicholas V.  The conclave was the first in the Apostolic Palace, the site of all but five papal conclave thereafter.  The conclave was also the first to feature accessus voting (votes cast "in accessit"), derived from a practice of the Roman Senate, where a cardinal could change their vote after an unsuccessful scrutiny to any cardinal already receiving votes.

Papal conclave, 2013: The Papal conclave of 2013 was convened to elect a pope to succeed Pope Benedict XVI following his resignation on 28 February 2013.  After the 115 participating cardinal-electors gathered, they set 12 March 2013 as the beginning of the conclave.  White smoke was seen emanating from the Sistine Chapel chimney on 13 March at 19:06 (7:06 PM) local time, following the fifth ballot, and the bells began pealing minutes after, signifying the election of a new pope.  The conclave elected Jorge Mario Bergoglio, SJ, then Archbishop of Buenos Aires, as pope.  He took the pontifical name of "Francis".

Papal conclave, 1667: The papal conclave, 1667 was convened on the death of Pope Alexander VII and ended with the election of Giulio Rospigliosi as Pope Clement IX.  The conclave was dominated by factions loyal to the cardinal nephews of Alexander VII and Urban VIII.  It saw the continued existence of the "Squadrone Volante", or "Flying Squadron", that had emerged in the 1655 conclave.  The conclave also saw Spain and France, the two largest Catholic powers at the time, both support Rospigliosi's election as pope.  Ultimately, Rospigliosi's election was achieved when the French ambassador bribed Flavio Chigi, Alexander's nephew, to support Rospigliosi.  Following the conclave all the parties believed that they had elected the pope that they had wanted.

Papal conclave, May 1605: The papal conclave of May 1605 was convened on the death of Pope Leo XI and ended with the election of Camillo Borghese as Pope Paul V.  This was the second conclave of 1605, with the one that had elected Leo XI having concluded just 37 days earlier.  It is significant for being the only recorded case of an injury at a papal conclave, which was the result of a physical fight amongst the cardinals about who should be elected pope.

Papal conclave, 1294: The papal conclave of December 23–24, 1294 was convoked in Naples after the resignation of Pope Celestine V on 13 December 1294.  Celestine V had only months earlier restored the Constitution "Ubi periculum" of Pope Gregory X, which had been suspended by Pope Adrian V in July 1276.  From that time every papal election has been a papal conclave.  It is the first papal conclave held during the lifetime of the preceding pontiff, the second such occurrence taking place in 2013 following the resignation of Pope Benedict XVI.

Papal conclave, 1700: The conclave of 1700 was a papal conclave convened following the death of Pope Innocent XII.  It ended in the election of Giovanni Albani as Pope Clement XI.  The conclave saw a rise in the dominance of the "zelanti" faction College of Cardinals.  It remained deadlocked for a month until the death of the childless Charles II of Spain.  The cardinal electors anticipated that his death without a clear heir would cause a political crisis, and moved to elect a pope that was seen as non-partisan.

Papal conclave, October 1503: The papal conclave of October 1503 elected Giuliano della Rovere as Pope Julius II to succeed Pope Pius III.  The conclave took place during the Italian Wars barely a month after the papal conclave, September 1503, and none of the electors had travelled far enough from Rome to miss the conclave.  The number of participating cardinals was thirty-eight, the College of Cardinals having been reduced by the election of Piccolomini as Pius III, who did not elevate cardinals.  At a consistory on October 11, Pope Pius had proposed to make Cardinal d'Amboise's nephew a cardinal, as part of his effort to conciliate the French, but the response from the Cardinals was not enthusiastic.

Papal conclave, 2005: The papal conclave of 2005 was convened to elect a new pope following the death of Pope John Paul II on 2 April 2005.  After his death, the cardinals of the Roman Catholic Church who were in Rome met and set a date for the beginning of the conclave to elect his successor.  Of the 117 eligible members of the College of Cardinals, those younger than 80 years of age at the time of the death of Pope John Paul II, all but two attended.  After several days of private meetings attended by both cardinal electors and non-voting cardinals, the conclave began on 18 April 2005.  It ended the following day after four ballots with the election of Cardinal Joseph Ratzinger, Prefect of the Congregation for the Doctrine of the Faith.  After accepting his election, he took the pontifical name of "Benedict XVI".

Resignation of Pope Benedict XVI: The resignation of Pope Benedict XVI occurred on 28 February 2013 at 20:00 (8:00 PM) CET (19:00 UTC).  The resignation was first announced on the morning of 11 February 2013 by the Vatican.  Benedict XVI's decision to step down as leader of the Catholic Church made him the first pope to relinquish the office since Gregory XII in 1415 (who did so in order to end the Western Schism), and the first to do so on his own initiative since Celestine V in 1294.  The move was unexpected, given that popes in the modern era have held the position from election until death.  The Pope stated that the reason for his decision was his declining health due to old age.  The conclave to select his successor began on 12 March 2013 and elected Cardinal Jorge Mario Bergoglio, Archbishop of Buenos Aires, Argentina, who took the name of Francis.

Papal conclave, 1549–50: The papal conclave from November 29, 1549, to February 7, 1550, convened after the death of Pope Paul III and eventually elected Giovanni Del Monte to the papacy as Pope Julius III.  It was the second-longest papal conclave of the 16th century, and (at the time) the largest papal conclave in history in terms of the number of cardinal electors.  The cardinal electors (who at one point totalled fifty-one) were roughly divided between the factions of Henry II of France, Charles V, Holy Roman Emperor, and Alessandro Farnese, the cardinal-nephew of Paul III.

Supporting paragraphs: Papal conclave, 2013 Resignation of Pope Benedict XVI
Answer: 11 February 2013