Pope Benedict XVI's final Sunday prayers in St Peter's Square will signal the start of a week in the Vatican that will make history with the first voluntary papal resignation in more than 700 years.
Tens of thousands of pilgrims have gathered in Rome from around the world following the ageing pope's shock announcement that he would step down because he lacked the strength of body and mind to govern the Church in modern times.
Crowds are expected to turn out in the Vatican on Sunday and again on Wednesday when the leader of the world's 1.2 billion Catholics will bid a final farewell to ordinary Catholic faithful from the Vatican's 16th-century plaza.
On Thursday, the pope will meet with his cardinals in the morning and take a helicopter to the papal summer residence of Castel Gandolfo in the afternoon.
There he will wave from his window and then retire for prayer in a chapel.
At 1900 GMT that same day, Benedict will become only the second pope in the Church's 2,000-year history to resign because he could no longer carry on.
Benedict has said he will live "hidden from the world" -- first at Castel Gandolfo and then in April or May, once renovations are complete, in a former monastery within the Vatican walls on a hillside overlooking St Peter's.
The Vatican has said the pope's divine infallibility -- the controversial power of making unquestionable utterances on matters of doctrine -- will abandon him the moment he is no longer sovereign of the Universal Church.
The resignation is a revolutionary act that many Vatican experts say could set a precedent for future popes when their energies inevitably begin to flag.
It has also caused dismay and in some cases disappointment among many ordinary Catholics who expect popes to serve until their natural deaths.
The only other pope to resign in similar circumstances was Celestine V, a hermit who stepped down just months after being elected in 1294 because he said he was not physically able and was disgusted with the intrigue of Rome.
In 1415, Gregory XII was forced out as part of a diplomatic deal to end the "Western Schism", when two rival claimants declared themselves pope.
-- Uncharted waters for the Church --
Quite unprecedented is the idea of a pope and his predecessor living within a stone's throw of each other inside the Vatican walls -- uncharted waters for the Catholic Church which will require sensitive handling.
The surprise has been so great that the Vatican has still not determined what titles Joseph Ratzinger will have -- "Bishop Emeritus of Rome" has been mooted -- and whether he can continue to wear papal whites.
The theologian pope wants to continue his academic research but Vatican spokesman Federico Lombardi has said he could provide "spiritual guidance" for his successor.
The Vatican's culture minister, Gianfranco Ravasi, even likened the image of the former pope in his hilltop monastery to the Biblical figure of Moses interceding with God for the Israelites as battles raged in the valley below.
Observers say the new arrangement could create confusion for many Catholics.
Immediately after the resignation, cardinals will hold conferences known as "congregations" starting on Friday that are a way of identifying priorities for the global Catholic Church and sussing out viable candidates for the papacy.
In the interim before the election of a new pope, the Church will be run by Cardinal Tarcisio Bertone -- a divisive figure who has been Benedict's right hand man for years and holds a post known as the "Camerlengo" ("Chamberlain").
Bertone will have the responsibility of disposing of the "Fisherman's Ring" -- a personalised gold signet ring that is a papal symbol and was traditionally destroyed after a pope's death to prevent his seal being put on documents.
Bertone and the cardinals will then set a date for the start of a conclave that is expected to bring together from around the world 117 "cardinal electors" -- cardinals below the age limit of 80 -- to elect a new pope.
No clear favourite has emerged although the pope's announcement indicates the need for a younger pope who can engage more with the modern world. Vatican watchers say the new pope should also be a more pastoral, less academic figure.
Under the normal rules following the death of a pope, the conclave has to start between 15 and 20 days after the pope passes away but Vatican canon lawyers are hard at work to see if the timeline can be brought forward.
The cardinals meet behind closed doors under Michelangelo's spectacular frescos in the Sistine Chapel in the Vatican and their deliberations are carried out under a strict vow of secrecy on pain of excommunication.
A conclave can last for several days before any candidate wins a two-thirds majority, after which the nominee is asked if he accepts the papal office and the ballots are burnt in a special stove that belches out white smoke.
The new pope then retreats to a special "Room of Tears" next to the Sistine Chapel where he dons the papal vestments and emerges onto a balcony over St Peter's Square to the Latin cry of "Habemus Papam!" ("We have a pope!")