They will dress in their traditional scarlet cassocks and hats, and, after a Mass and lunch, will file one by one into the 15th-century sanctuary, chanting an ancient ode to God.
They will have surrendered cell phones, radios and any other links to the outside, and they will swear on the Gospels never to speak of the proceedings about to transpire. Once inside, the cardinals will hear speeches from Vatican officials describing the state of the church and their immediate duties, then they will dismiss the speakers and all others not eligible to vote.
A cardinal will rise, walk to the chapel door and lock it, and the conclave will officially begin.
"What it comes down to is that these cardinals are looking for someone who can serve as a successor to [St.] Peter," said John Wauck, a Vatican expert teaching at Rome's Pontifical University of the Holy Cross. "They are looking for someone they can imagine calling 'your holiness,' the next vicar of Christ, servant of the servants of God."
"Frankly, anybody who wants to be pope is out of his mind," said Archbishop John P. Foley, president of the Vatican's Pontifical Council for Social Communications, and who has worked at the Vatican for 21 years. "It's a living martyrdom."
The group of cardinals in the conclave will be the largest and most geographically diverse since the Church, in 1179, recognized cardinals as the sole electors of a pope. Although 117 cardinals are eligible to participate, two are unable to travel because of illness.
Though only two of the men present participated in the conclave that chose Pope John Paul II, the cardinals will all know what is expected.
Three cardinals, chosen at random, will count the ballots, and one will read the names aloud if the number of votes corresponds to the number of cardinals present. As each ballot is read, the cardinal will pierce it with a needle and thread the ballots together, a tradition designed to ensure that none of them is lost or saved. A second set of three cardinals then recounts the ballots.
The cardinals will continue voting four times a day - twice in the morning, twice in the evening - until that two-thirds majority is reached. If no pope is elected after the third day, they may choose to rest for a day of reflection before resuming the schedule, taking an additional rest after each group of seven unsuccessful ballots.
The pattern can continue twice more - seven votes, optional break, seven votes, optional break - before a provision created by Pope John Paul allows the cardinals to decide that a simple majority of the ballots will be enough to elect a pope.