The Vatican press office said the decision was made during a vote Friday of the College of Cardinals. Tuesday will begin with a Mass in the morning, followed by the first balloting in the afternoon.
Earlier Friday, the Rev. Federico Lombardi said the start date of the conclave will likely be either Monday, Tuesday or Wednesday.
A few hours later, Cardinal Roger Mahony of Los Angeles tweeted, "I hope we vote this evening to set the starting date for the Conclave; preceded by the Mass to Elect a new Pope. Prayers, please!!"
The last cardinal arrived in Rome Thursday. All 115 voting cardinals are trying to get to know each other. There are some interesting statistics in regards to the 115 cardinals. They aren't young; their average age is between 71 and 72 years old. Not surprisingly, the country most represented is Italy with 28. The United States is in second place with 11.
There doesn't appear to be a frontrunner in the election. But For the first time ever there is speculation that Americans have a chance to be pope. An Italian magazine is reporting there are two American contenders, Cardinal Timothy Dolan from New York City and Boston's Sean Patrick O'Malley.
Cardinal Dolan is very big on evangelization and has been a great spokesperson for the church. Cardinal O'Malley has gotten high marks for dealing with the sex abuse crisis in three dioceses that he's worked.
This week's pre-conclave discussions have exposed divisions among cardinals about problems the church is facing, including how to govern the Vatican itself. Much of the discussion in the meetings is thought to be about corruption in the Vatican in addition to the priest abuse scandal. According to New York Cardinal Timothy Dolan's blog, cardinals are also discussing preaching and teaching the faith, Catholic schools, hospitals and protecting the unborn. Dolan calls these the big issues.
Meantime, it's believed that the cardinals will move into their new digs at the Vatican's Santa Marta Hotel on Monday afternoon. That's where they'll live while sequestered for the conclave. The cardinals will participate in a drawing for the rooms. The hotel also contains the room where the newly elected pope will live for the first few weeks while the papal apartments are being renovated.
Monsignor Terry Fleming, of the Mid-Wilshire District, says local Catholics are keeping a close eye on this historic process.
"The issues of Los Angeles are no different from the issues of the world. Whatever the church is going through in Los Angeles, it's going somewhere else," Fleming said.
The cardinals have been attending pre-conclave meetings to discuss the problems of the church and decide who among them is best suited to fix them as pope.
Fleming predicts this will be a longer conclave. In the past 100 years, no conclave has lasted longer than five days.
A two-thirds majority is required to pick the next pope. In 2005, they only needed a simple majority.
The Associated Press contributed to this report.