Being named a saint is, usually, the end of a four-step process.
First, after the candidate's death, someone needs to begin gathering the evidence to support sainthood, and then present that to the Vatican's Congregation for the Causes of Saints.
Once the congregation has the evidence, it evaluates the candidate. If the congregation finds that the candidate lived a life of heroic virtue, it turns the candidate's name over to the Pope who proclaims the candidate "venerable."
The next step is the investigation of possible miracles attributed to the candidate. These usually involve the healing of someone who prayed to the candidate. If the cure defies medical explanation by a panel of doctors, theologians and high level church officials, the congregation can declare it to be a miracle and forward the candidate's name to the pope for beatification. Once beatified, the candidate can be called "blessed."
It takes a second miracle to be canonized... usually. After canonization, the candidate can be called "saint."
But, there are exceptions.
Elected in 1958, Pope John XXIII was known as "The Good Pope." He convened Vatican II, which led to sweeping reforms and modernizations within the church. He spoke to the faithful as a minister, not as an aloof leader. He was seen, with the help of the new medium of television, as a paternal man who wanted the church to become a leader in world peace.
Pope John XXIII does not, however, have two miracles attributed to him.
He was beatified in 2000 by Pope John Paul II after the Vatican determined that he healed a nun who was on the edge of death, suffering from a gastric hemorrhage. Pope Francis has waived the need for a second proven miracle, because he wanted John XXIII to be canonized alongside John Paul II. In making that decision, Pope Francis acknowledged that the faithful already recognize John XXIII as a saint, because of his life and his works.
Pope John Paul II was beatified in 2011, just 6 years after his death. Here too, rules were bent. There is typically a 5-year waiting period after the death of a candidate, before any of the process can begin. Pope Benedict waived that rule for John Paul II. In 2011, the Vatican concluded that John Paul II healed a French nun suffering from Parkinson's disease.
His second miracle was the healing of a Costa Rican woman whose brain aneurism disappeared after she prayed to him, on the day of his beatification.
When Sunday's canonization takes place, not only will it be the first time two popes have been declared saints, it will be the first time two living popes will there for the ceremony.
With contributions from the Associated Press