PHILADELPHIA (AP) -- Pope Francis' six-day, three-city tour of the U.S. this week evolved from a pledge he made last fall to attend the World Meeting of Families in Philadelphia. Stops in Washington and New York were added after Francis put the triennial, Vatican-sponsored conference on his agenda. Here are answers to key questions about the event opening Tuesday that's spurring Francis' first-ever visit to America and the City of Brotherly Love:
What is the World Meeting of Families?
Organizers describe the conference blending prayer, religious instruction and faith-themed lectures, as the world's largest gathering of Catholic families.
With more than 18,000 people signed up, this year's will be the most attended of the eight World Meetings. That's double the registrations for the last World Meeting, in Milan in 2012.
The World Meeting of Families was conceived by Pope St. John Paul II in 1992 (The first one wasn't held until 1994.) He started the event to explore family bonds and the role of families in society. He presided over the inaugural edition in Rome during the UN's International Year of the Family.
More than 100 countries will be represented this year, with the highest number of registrants coming from the United States, Canada, Mexico, Pakistan and Vietnam.
What are the notable sessions?
Boston Archbishop Cardinal Sean Patrick O'Malley and evangelical pastor Rick Warren are teaming up for a talk on living a life of faith and joy and trusting in God's plan.
Los Angeles Archbishop Jose Horacio Gomez is speaking on the impact of the U.S. immigration system on families and children and calling for reforms to protect human rights.
Breakout sessions cover a variety of topics tied to family and relationships, including interfaith marriage, divorce, dating, the "hook-up culture," sexuality, reproduction and infertility.
"There's No Vacation from Vocation" covers the role of God's will in day-to-day decisions. "Loving on the Edge" deals with using faith to heal the pain of damaged relationships.
The conference also has a youth track, with musical performances and activities.
Does the pope always attend?
Pope St. John Paul II celebrated Mass at the first three World Meetings, in 1994 and 2000 in Rome and in 1997 in Rio de Janeiro. He appeared via a live broadcast at the 2003 event in Manila.
Pope Benedict XVI celebrated Mass at the 2006 World Meeting in Valencia, Spain and in 2012 in Milan. He appeared via satellite at the event in Mexico City in 2009.
What is Pope Francis' role?
He'll attend the World Meeting's closing concert, the Festival of Families, on Saturday and celebrate Mass on Sunday. Both events are being held on the city's Benjamin Franklin Parkway.
Francis will parade down the parkway's outer lanes before both events.
At the festival, he'll hear families from around the world describe the joys and challenges accompanying their family lives.
Organizers expect more than 500,000 people at the festival and more than a million people at the Mass.
People attending the World Meeting of Families are automatically ticketed for an upfront section at both events.
How was Philadelphia selected?
Benedict XVI chose the city at the 2012 World Meeting in Milan despite the Archdiocese of Philadelphia's initial reluctance to offer itself as a host.
The Catholic Church in Philadelphia was reeling at the time from a clergy sex abuse scandal, a $17 million budget deficit and $300 million in debt. The archdiocese's teachers had gone on strike and an administrator was charged with embezzling nearly $1 million.
Archbishop Charles Chaput was stunned. Now, four years removed from those dark days, he sees the World Meeting as a godsend.
"The city of Philadelphia and the church in Philadelphia are major players in the story of our country. They deserve better than the problems of the last decade," Chaput said recently. "And they deserve some joy. They deserve a win, and a turnaround moment that renews the spirit. And I think that's why the Holy Spirit guided Pope Benedict XVI to choose Philadelphia as the place for this World Meeting of Families."