After the resignation of Pope Celestine V in 1294, Pope Boniface VIII inserted into the corpus of canon law a provision for the resignation of a pope. When ecclesiastical law was organized into a Code of Canon Law in 1917, this law was included. It likewise passed into the 1983 Code of Canon Law promulgated by Pope John Paul II.
Canon 332, 2 states: If it should happen that the Roman Pontiff resigns his office, it is required for validity that he makes the resignation freely and that it be duly manifested, but not that it be accepted by anyone.
This canon applies the general principles governing resignations from office (cf. canons 187-189) to the special case of the Roman Pontiff. Since a Pope is not subject to any earthly authority, including the College of Cardinals and the Apostolic College (the Bishops), there is no one who need, or even can, accept it, refuse to accept it, or change the character of its terms. It need only be freely made as an interior act of will (without force or coercion) and duly manifested to the Church.
Pope Benedict XVI
On Monday 11 February 2013, Pope Benedict XVI publicly manifested his decision to lay down the responsibilities of the papal office on Thursday 28 February at 8 p.m. Rome time (2 pm Eastern). On that day he will travel to Castel Gondolfo, the papal summer palace in the Alban Hills, south of Rome, returning at a future date to live in the Mater Ecclesiae Monastery contained within the grounds of Vatican City.
The Holy See has also announced that the current pontiff will play no role in the conclave to elect his successor. That process is governed by Universi Dominici Gregis, the special law on electing a pope. It requires that the Conclave begin on the 15th day after the February 28th vacancy (or March 15th). For serious reasons the Cardinals may choose to postpone as late as the 20th day (March 20th). Unless Pope Benedict sets a different date, while still Pope, this law will determine the beginning of the Conclave to elect Benedict's successor.