(CNN) -- Pope Emeritus Benedict XVI will attend the Mass of canonization of John Paul II and John XXIII, the Holy See confirmed Saturday, according to Vatican Radio.
Benedict XVI, who will be attending at the invitation of Pope Francis, will not be at the altar, but will figure among the cardinals and bishops.
It won't be the first time Benedict has made a public appearance since taking the almost unprecedented step of resigning the papacy.
He was also present for a ceremony in February in which Francis created 19 new cardinals.
Since a new pope usually takes the reins only following the death of his predecessor, seeing both him and Francis together remains a highly unusual event.
The two canonization candidates share an improbable path to sainthood: they both rose from very humble beginnings to lead the Roman Catholic Church.
John XXIII (1881-1963) -- known as Angelo Giuseppe Roncalli before he became Pope -- was one of 13 children born into a family of Italian peasants, farmers from a tiny village in the country's north, before being sent away to study for the priesthood at age 11.
John Paul II (1920-2005), born Karol Jozef Wojtyla, was brought up in a grimy industrial town in Poland and raised by his soldier father after his mother died when he was just 8-years-old. He spent his formative years living under first Nazis, then Communists.
His beatification is the quickest in modern times, made possible because Benedict -- who succeeded John Paul in 2005 -- waived the normal five-year waiting period after death to get someone's beatification rolling.
Vatican spokesman Father Federico Lombardi said as many as 150 cardinals and 1,000 bishops would attend the canonization ceremony, as well as 24 heads of state, Vatican Radio reported.
The ceremony is also expected to draw upward of a million pilgrims, who will gather in St. Peter's Square to witness the two former popes enter the celestial community of Catholic saints.
CNN's Laura Smith-Spark, John L. Allen, Jr. and Daniel Burke contributed to this report.