In what may also be signposts into the pope's wider focus, the Vatican announced Monday that the events surrounding the Mass will see:
- Combined prayer between the pontiff and representatives of the Eastern Rite churches at the tomb of St. Peter;
- Proclamation of the Gospel being made in Greek, "in order to simplify the liturgy";
- The pope wearing a simple new ring of office, a gold-plated silver band, recycled from one owned by Pope Paul VI's private secretary; and
- Adoption of the Latin phrase Miserando atque eligendo as the pope's motto, a reflection on Christ's mercy in choosing Matthew, who was a tax collector, as an apostle.
Official estimates for the pope's first Angelus blessing, held at noon Sunday, put the number there at about 300,000.
Representatives of governments from around the world are expected to attend Tuesday. U.S. Vice President Joe Biden arrived in Rome on Sunday night for the event.
News of the pope's plans for the event came Monday at a Vatican press conference.
The pope is to pray with the Eastern Rite church leaders before the Mass begins at the crypt of St. Peter, where the apostle is thought to be buried and upon which St. Peter's Basilica is built.
Included in the group will be the ecumenical patriarch of Constantinople, Bartholomew I, the first of his line to attend a papal inauguration since the year 1054, when the Eastern and Western churches split.
Following the prayer, the official ceremony will get under way with the pope receiving both his ring -- known as the fisherman's ring, as Peter was a fisherman before an apostle -- and his pallium, a woolen liturgical vestment signifying unity with the world's archbishops and the pope's role as shepherd of the universal church.
The pope will be using the same pallium as his predecessor, Pope Benedict XVI, the Vatican spokesmen said Monday.
The ring he will wear is from a ring worn by Pope Paul VI's private secretary, Archbishop Pasquale Macchi, the spokesmen said. Unlike Benedict's ring, which was made of gold, Francis' will be made of gold-plated silver.
A picture of the ring provided by the Vatican shows a small face with a relief image of a bearded St. Peter holding the symbolic keys to heaven.
Clarifying the ring's lineage, the Vatican issued a press release late Monday afternoon saying that the ring "was in the possession of Archbishop Macchi, Pope Paul VI's personal secretary" and then had been owned by another priest before being proposed as a papal ring to Francis by Cardinal Battista Re, a former prefect of the Vatican's Congregation for bishops.
The pope has also approved his official coat of arms, using the same heraldry he used while he was bishop of Buenos Aires, Argentina, the Vatican said Monday.
The coat of arms features a blue background with three symbols focused on the Holy Family: the official seal of the Jesuits, which spells out IHS, the first three letters of Jesus' name in Greek; an image of a spikenard flower, traditionally a symbol of Joseph; and a five-pointed star, a symbol of Mary.
The pope's motto, which he also used in Argentina, is taken from a homily of St. Bede, an eighth-century English monk.
Translated literally, it means, "because he saw him through the eyes of mercy and chose him."
"Jesus saw Matthew, not merely in the usual sense, but more significantly with his merciful understanding of men," St. Bede wrote in the homily the motto references.
Tuesday's inaugural Mass is being held on the feast day of St. Joseph. The pope has decided to use the normal readings for the day instead of special readings for the occasion, the Vatican said Monday.
The first reading will be made in English; the second in Spanish. The pope will homilize in Italian.
Although Basilian Fr. Thomas Rosica, who provides English translation during the press conferences, said a text of the homily would be available to journalists ahead of time, he also said, "As we have noticed over the past few days, there is a certain spontaneity in the pope's talks."
The first reading for the Mass will be from the Book of Samuel, recalling the Lord speaking to the prophet Nathan, asking him to tell David that "I will preserve the offspring of your body after you and make his sovereignty secure."
The second reading will be from Paul's letter to the Romans, regarding God's promise to Abraham "on account of the righteousness which consists in faith."
The Gospel account, taken from Matthew, focuses on Joseph's uncertainty around Mary's unexplained pregnancy. Appearing before Joseph, according to the account, an angel tells Joseph to stay with Mary regardless.
Among the petitions are: prayers for government officials, that they may "help build the civilization of love"; and prayers for the poor, that God will "grant them refreshment, comfort, and hope, not least through the love of their brothers and sisters."
A total of 132 delegations from around the world are expected to attend the event, the Vatican said, including 31 heads of state and two ruling monarchs.
Stressing that the Vatican is not controlling the guest list -- which is reported to include controversial Zimbabwean president Robert Mugabe -- Rosica said all are welcome to attend.
"The Holy See does not invite any foreign delegation ... we inform the world that this is taking place," Rosica said. "Those who wish to come are welcome, no one is refused."
On Wednesday, Francis will meet with representatives of both Christian groups and those of other faiths, the Vatican said.
Stressing the tie between ecumenism, or relations between Christian churches, and relations between other faiths, the Vatican said the pope would be meeting with the ecumenical and interfaith leaders at the "same moment, not two different moments."
Among those included in the meeting will be leaders of 33 Christian churches, 16 Jewish leaders, and leaders of the Muslim, Buddhist, Sikh and Jain communities.
Included in the Christian group will be Bartholomew I; Metropolitan Hilarion of the Patriarchate of Moscow; a representative of the Armenian orthodox church; an Anglican representative; and the secretary of the World Council of Churches.