What is the World Meeting of Families?

CENTER CITY (WPVI) -- The World Meeting of Families, which culminates with a visit by Pope Francis this weekend, begins today in Philadelphia.

But what is it exactly? And who will foot the bill for the many preparations being made for the papal's visit?

Organizers describe the conference that blends 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.

And with the big event comes big costs and spending.

City officials said they finalized a contract Tuesday with the World Meeting of Families.

Based on the contract, the organizers of the event will reimburse the city $12 million for monies spent on preparations for the pope's visit.

Of those funds, nearly $5 million is for law enforcement and nearly $4 million is for the fire department.

The World Meeting of Families was conceived by 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.

The conference is held every three years and is sponsored by the Holy See's Pontifical Council for the Family. The events will be held at the Pennsylvania Convention Center on Arch Street in the city.

The event ends with a closing ceremony at 1 p.m. Friday afternoon.

"Each World Meeting of Families has a theme that energizes and enlivens the event while adding great depth of meaning to our understanding of families. The theme of the World Meeting of Families - Philadelphia 2015 is "Love Is Our Mission: The Family Fully Alive," emphasizing the impact of the love and life of families on our society," the WMOF website reads.

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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 Jos 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.

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Does the pope always attend?

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.

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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 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.

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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."

Many of the people Action News talked to planned to stay through the weekend, and for the papal visit.

"I think it's a good idea for the pope coming here to America and all its atmosphere and open hearts," said Maria Stasch from Germany.

"Actually when I first registered I didn't know the pope was coming. That was an added on bonus later," said Sister Valerie from Sydney, Australia.

"We're going to be here for the whole week, with some of the sisters from my convent, and some of our other sisters from the local convent in Philadelphia," said Sister Rita Ashray of Monroe, CT.

The Sturgeon family in from Ohio wants the full experience; they've already taken a tour bus to all of the city's main attractions, something city leaders are hoping other visitors do too.

"It's a wonderful city. I [said] we have to come back when we have more time ad there's not as many people," Mary Sturgeon said.

For more information about the event, visit WorldMeeting2015.org

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The Associated Press contributed to this report.