VATICAN CITY -- In a historic match-up of papacies past and present, Pope Francis and emeritus Pope Benedict XVI will honour Popes John XXIII and John Paul II in the first ever canonization of two popes. For such a momentous occasion, the ceremony is actually quite short and surprisingly straightforward, with the added benefit that the relics of the two new saints will be presented for universal veneration by the church for the first time. Here are five things to look for -- though beware, the rite is celebrated mostly in Latin, the official language of the Catholic Church.

1. THE CEREMONY

The preliminary part of the ceremony begins at around 0700 GMT with prayers, hymns and culminates with the chanting of the hypnotic Litany of Saints, the roll call of the church's saints, each one followed by the refrain "Ora pro nobis," or "Pray for us."

After Pope Francis and concelebrants process to the altar, the canonization rite begins immediately.

The head of the Vatican's saint-making office, Cardinal Angelo Amato, asks the pope three separate times to include John XXIII and John Paul II among the saints. In the rite for beatification, there is only one such petition. The three repeated requests for canonization "signify the importance of this celebration," noted the Vatican spokesman the Rev. Federico Lombardi.

2. THE PRONOUNCEMENT

Pope Francis then says: "For the honour of the Blessed Trinity, the exaltation of the Catholic faith and the increase of the Christian life, by the authority of our Lord Jesus Christ, and of the holy Apostles Peter and Paul, and our own, after due deliberation and frequent prayer for divine assistance, and having sought the counsel of many of our brother bishops, we declare and define Blessed John XXIII and John Paul II be saints and we enrol them among the saints, decreeing that they are to be venerated as such by the whole church. In the name of the Holy Father, and of the Son, and of the Holy Spirit."

3. THE RELICS

Relics of the two new saints are then brought to the altar. In the case of John Paul, the same relic -- his blood -- used for his 2011 beatification is being used. For John XXIII, a small piece of skin taken after his body was exhumed for his 2000 beatification is being used. Relics -- the physical remains of a new saint or things he or she touched in life -- are used to help the faithful venerate.

Amato then thanks Francis and asks him to draw up an official document attesting to the canonization. Francis responds "We so decree" and the rite ends with the singing of the "Gloria."

The Mass then proceeds as usual.

4. BY THE NUMBERS

--The Vatican said Saturday that Francis would preside over the Mass and Benedict would concelebrate along with 150 cardinals and 700 bishops. It's the first time Benedict has joined Francis in celebrating Mass in public since his resignation in 2013. Benedict attended the February ceremony in St. Peter's Basilica in which Francis installed 19 new cardinals, but as a spectator.

--About 600 priests will distribute Communion in St. Peter's Square and 210 deacons will distribute Communion to the throngs of people expected to line up along Via della Conciliazione, the main boulevard leading away from the square.

--93 official delegations attending, including an estimated 24 heads of state. The kings and queens of Belgium and Spain are expected, as are royals from Andorra, Britain and Luxemburg. Poland is sending one of the largest delegations with the current president and two former presidents, including Lech Walesa, founder of the Solidarity movement that toppled communism in Poland, which John Paul supported.

--20 Jewish leaders from the U.S., Israel, Argentina, Poland and Rome.

5. VIRTUAL CANONIZATION

--Official website www.2popesaints.org with information also available at www.vatican.va

--Vatican liturgical booklet with step-by-step process of canonization rite and order of Mass at http://www.vatican.va/news--services/liturgy/libretti/2014/20140427-l ibretto-canonizzazione.pdf

--Official hashtag #2popesaints