It will be bigger by far than the GOP convention, which filled the city in 2000.
Exactly how big, no one yet knows.
Mayor Nutter and Archbishop Charles J. Chaput on Monday formally announced plans for Philadelphia to host the World Meeting of Families, a Catholic gathering expected to bring the next pope here in 2015.
It was the first question to Chaput at a midmorning news conference: Will the new pope, expected to be chosen in March, come to the city?
"I can't imagine that he won't, but I can't promise that he will," Chaput said at the Philadelphia Archdiocese in Center City. "We're going to plan as though he's going to come."
Mayor Nutter pledged that the city would be ready and eager to perform on an international stage.
"We are more than prepared to logistically handle something like this," Nutter said. "We have every confidence we'll be able to pull this off in spectacular fashion."
It will be the first World Meeting of Families, also called World Family Day, in the United States. The once-every-three-years event requires gigantic preparation and fund-raising, the latter a challenge to the Philadelphia Archdiocese.
The most recent day, in Milan, Italy, cost about $13.3 million, Chaput said.
How will fund-raising proceed here?
"I'm going to stand at the door and ask everyone for a gift," Chaput joked, bringing laughter from about 35 reporters, church leaders, and government officials.
More seriously, he said funding will be sought inside and outside Philadelphia.
Nutter said, "I know a little bit about raising money, have been known to pick up the phone."
Chaput then said, "We're consciously aware of the separation of church and state" - at which point Nutter playfully put his hands on the archbishop's shoulders and said, "I wanted to leave that one to you."
On the streets of Philadelphia, Catholics reacted positively to news of the event, even knowing it is two years away.
"There's a lot of work to be done in family life today," said Sister Jean Crane of the Sisters of St. Joseph, on 10th Street near Citizens Bank Park. "I hope people come out. I hope people would heed the words that are going to be brought to them."
On Christian Street near Ninth, down the block from St. Paul parish, Joe Lomanno, 68, owner of Superior Pasta Co., said he remembered taking his son to see Pope John Paul II in 1979 when his boy was 4 or 5.
"It was a thrill," he said.
Lomanno said he hoped the new pope would be Cardinal Timothy Dolan of New York - "Conservative, but not to the extreme" - but said he would try to see whoever is chosen as pope during the Philadelphia event. A papal visit would be good for younger Catholics who, he said, are mostly absent from the Masses he attends every Sunday.
"It will be positive," he said. "Especially if it's a charismatic pope."
At 10th and Ellsworth Street, outside Annunciation of the Blessed Virgin Mary church, Damaris Tellez, 26, remembered the excitement that accompanied John Paul's visit to Mexico City in 1999.
"It will be a big inspiration for all of us who are still believers," she said.
Gov. Corbett, who, with Nutter, will serve as honorary cochair of the event, issued a statement that he was delighted at Philadelphia's selection.
"Today across Pennsylvania, our places of worship are full of strong, persevering people of faith," he said, calling the event "an opportunity to celebrate the religious heritage and freedom that have built both our state and our nation."
The event will be from Sept. 22 to 27, 2015. It is expected that Nutter, Corbett, and Chaput will travel to Rome in spring or summer to meet with Vatican planners.
The local archdiocese has struggled with a clergy sex-abuse scandal, budget problems, school closings, and parish mergers. The pope's arrival would bring not just optimism and pride, but worldwide attention and gigantic attendance.
In 1979, John Paul II's outdoor Mass on the Benjamin Franklin Parkway was estimated to have drawn one million people. It is expected that the next pope could draw at least as many, given the growth in infrastructure, transportation, and communications.
Philadelphia has 10,800 hotel rooms in Center City, part of 34,000 in the five-county region of Philadelphia, Delaware, Montgomery, Chester, Montgomery and Bucks, according to the Greater Philadelphia Tourism Marketing Corp.
The event was established by John Paul II in 1992 to strengthen the bonds of marriage and family. It includes seminars on religion, marriage, divorce, and other topics.
Pope Benedict XVI drew one million to an open-air Mass at the Milan gathering. Benedict, who shocked the world by saying he would resign this month, had named Philadelphia as the next site of the gathering and had planned to attend.
Contact Jeff Gammage at 215-854-2415, firstname.lastname@example.org, or on Twitter @JeffGammage.