As reported by The Associated Press, Pope Benedict XVI on Monday announced his resignation before cardinals at a ceremony to name three new saints.
According to a post on Poynter.org:
Many news organizations have reported that Pope Gregory XII was the last leader of the Roman Catholic Church to resign, in 1415. However, in this clip NBC Vatican analyst George Weigel says it’s been “717 years” since the last pope abdicated, by which he probably means Celestine V, who resigned 719 years ago, in 1294.
Gregory was not the pope for whom was named the Gregorian calendar (that was Gregory XIII) or Gregorian chant (Gregory the Great or Gregory II, opinions differ). XII resigned so the church could unite under a single pope after a schism. He died in 1417. Here’s some background on papal resignations.
Here is the pope's announcement. It has been translated into English:
I have convoked you to this Consistory, not only for the three canonizations, but also to communicate to you a decision of great importance for the life of the Church. After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry.
I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.
For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint 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 Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects. And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff. With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer.