Accompanied by the chant Vieni Creator and marching in solemn procession, 115 cardinal electors will this afternoon file into the Sistine Chapel in the Vatican for the opening rites of the conclave that will elect the successor to Pope John Paul II, writes Paddy Agnew in Rome.
Just 16 days after his death, the cardinals will come together to swear a solemn oath on the eve of an electoral process which is not only one of the oldest in the world but, on this occasion at least, one of the most difficult to predict.
No one can say whether the next pope will be European or Latin American, Italian or Asian, relatively young or older if this will be a short or long conclave.
Nor, following a statement from Vatican senior spokesman Dr Joaquin Navarro-Valls on Saturday, can we even be sure that the first vote will take place this evening.
The 1996 apostolic constitution, Universi Dominic Gregis, drawn up by Pope John Paul II to clarify the rules governing the election of a pontiff, stipulates that the cardinals may hold their first vote on the afternoon of the opening day.
This is because their day begins this morning with the votive Mass, Pro Eligendo Papa, in the basilica of St Peter.
Only after the Mass, in the afternoon, can the cardinals approach the practical business of voting. In the past, they have traditionally voted once on the opening afternoon.
However, Dr Navarro-Valls on Saturday suggested that it was by no means certain that there would be an initial vote this afternoon. Vatican watchers have immediately speculated that the uncertainty about today's vote mirrors a much greater uncertainty about the outcome.
It might be that the cardinals, rather than heading straight into a formal vote, might want to take a last few soundings among themselves.
Such soundings might eliminate the so-called stalking horse candidates, such as German Cardinal Joseph Ratzinger and Italian Cardinal Carlo Maria Martini.
Throughout the last week, there has been speculation that these two men, from radically different backgrounds and representing (to some extent) a curia and a non-curia choice, might figure in the first ballot as a way of testing the electoral waters.
Neither candidate is likely to attract the two-thirds majority (77 votes) necessary to win the election, yet a ballot held around them would indicate the feelings of the college of cardinals.
Those who vote for Cardinal Ratzinger (78), would be in favour by and large of an older, less dynamic, "safe pair of hands" pope who follows on in the doctrinally conservative tradition of John Paul II.
Those who vote for Cardinal Martini would favour a younger non-curia pope who might be Latin-American, who would emphasise greater collegiality in church affairs and vigorously promote Pope John Paul II's social teachings in relation to the globalisation debate.
Having tested the electoral waters, the choice may then focus on someone who could appeal to all sides of the conclave, men such as Argentine José Mario Bergoglio; Mexican Norberto Rivera Carrera; Portuguese José da Cruz Policarpo; Belgian Godfried Daneels; Indian Ivan Dias; Brazilian Claudio Hummes; Canadian Marc Quellet; Austrian Christoph Schoenborn, or possibly, someone whose name has yet to figure in media speculation.
Vatican technicians meanwhile have been busy "de-bugging" the Sistine Chapel and the Casa Santa Marta, where the cardinals will stay, to eliminate any surveillance devices placed to eavesdrop on the their conversations.