In 1997, then 19-year-old Jessica Smith sang on the papal stage at World Youth Day in Paris as pilgrims made their way to Longchamp Racecourse for the event's final Mass with Pope St. John Paul II. She was amazed by the sheer number of people, she said, and while her singing ministry led her to stages at other large Catholic events, World Youth Day stands apart in her memory.

"What was really transformative in my faith, as a result of World Youth Day, was that I finally understood the magisterium, like the reality of the papacy became known to me," said Smith, now 46, a pastoral associate at her parish in Columbia City, Indiana. "You couldn't be anywhere near John Paul II and not be transformed. … It connected me to the church in a more concrete way."

Knowing the impact the event had on her as a young adult, Smith and her husband are traveling with six of their 12 kids to Lisbon, Portugal, for World Youth Day, Aug. 1-6. She expects the sacrifices -- including a hefty financial cost -- required to make it possible for her oldest children to go to be worth it, she said.

"Young adulthood is such a time of transition, and it's really easy to get lukewarm about your faith as you're really busy with all the changes in life," she said. "I hope it (World Youth Day) draws them in close and helps them to feel really connected to Jesus."

Smith's kids, ages 16 to 22, will be among the 400,000 registered young adults and as many as 2 million general pilgrims anticipated to attend World Youth Day this year with Pope Francis. The event, a worldwide gathering historically held every two to three years, is a significant undertaking, requiring years of planning and tens of millions of dollars to prepare and host, with one member of the 2016 WYD Organizing Committee in Krakow comparing it to preparing for the Olympics, "only with a bigger audience and one absolutely important VIP guest” -- the pope.

Previous World Youth Days' attendance has ranged from hundreds of thousands to millions of young people, with the largest being WYD 1995 in Manila, which was estimated at 5 million. The journey to the event is frequently described as a pilgrimage, with the goals of growing spiritually and overcoming hardship, setting it apart from typical travel.

The event traces its roots to 1984, when St. John Paul held a youth jubilee at the Vatican for the Holy Year of the Redemption that drew 300,000 young people, mostly Italians. The following year, he invited young people back to St. Peter's Square for Palm Sunday to celebrate International Youth Year, as proclaimed by the United Nations. Again, an estimated 300,000 young people came. In December 1985, he established World Youth Day, an annual church observance that would also include periodic international gatherings.

The first official World Youth Day was organized the next year in the Eternal City with the theme, "Always be prepared to make a defense to anyone who calls you to account for the hope that is in you." In 1987, the gathering went international in Buenos Aires.

Buenos Aires was followed by Santiago de Compostela, Spain (1989); Czestochowa, Poland (1991); Denver (1993); Manila (1995); Paris (1997); Rome (2000); Toronto (2002); Cologne, Germany (2005); Sydney (2008); Madrid (2011); Rio de Janeiro (2013); Krakow, Poland (2016); and Panama City (2019).

"Undoubtedly, establishing World Youth Days was one of the most prophetic decisions of Saint John Paul II and nowadays the Church still benefits from its fruit," Cardinal Stanislaw Rylko, president emeritus of the Pontifical Council for the Laity, wrote in a 2016 book, "World Youth Days: A Testimony to the Hope of Young People."

"For many young people, WYDs were the moments of deep internal transformation or even authentic conversions," he said.

That was the case for Bishop Andrew H. Cozzens of Crookston, Minnesota, who was among the nearly 700,000 young people who descended upon his hometown of Denver when the city hosted World Youth Day.

"World Youth Day was a life-changing experience for me," he said, first, because he encountered the universal nature of the church and saw tens of thousands of young people engaged in Mass and going to confession, and second, because he encountered the pope.

As St. John PauI spoke to the pilgrims during the prayer vigil at Colorado's Cherry Creek State Park, the 25-year-old future priest and bishop left his group and, alone, weaved through the crowd to get closer to the pope. "I remember being so taken by John Paul as he was speaking that I just wanted to get as close as I could to him," Bishop Cozzens said. "I just remember thinking, 'I will follow you wherever you go' … especially the way he was inviting us to pursue holiness with our whole hearts."

World Youth Day not only changed the lives of participants, it also changed the church in Denver and the United States, Bishop Cozzens said.

"It's had a generational impact, like lots of people came back to the church after World Youth Day in Denver," he said. "In Denver, RCIA (Rite of Christian Initiation of Adults) classes were filled, and apostolates were born, and a lot of people of my generation experienced such a profound impact that they ended up giving their lives to the church in more profound ways."

Paul Jarzembowski, associate director for the laity in the U.S. Conference of Catholic Bishops' Secretariat of Laity, Marriage, Family Life and Youth, was among those young people at World Youth Day in Denver inspired to use his gifts to serve the church. Now Jarzembowski oversees World Youth Day planning for U.S. pilgrims on behalf of the U.S. bishops. As a teenager in Denver, he recalls St. John Paul telling the pilgrims to "proclaim the Gospel from the rooftops" and taking that message to heart.

"I kept imagining myself climbing up on the rooftops of my neighborhood," he said. "I thought, yeah, I can't just be kept in my house, in my school, in my small group of friends at my church. I need to take this beyond it. So it really introduced for me this idea that I needed to take responsibility for my faith life, which led me ultimately to a career working in the church."

Statistics support testimonials lauding the fruit of the event. Spanish social research company GAD3 has organized a survey among participants after every World Youth Day since Madrid 2011. After World Youth Day in Krakow 2016, they asked young people whether World Youth Day strengthened their relationship with God, and 98% said yes. The theme of WYD 2016 was "mercy," and 90% of the youth said the event taught them more about mercy, while 89% said WYD 2016 strengthened their relationship with the church.

There is also a bigger social perspective, as 97% of youth said, "WYD strengthened my will to improve society."

That was true for Kasia Wasiutynska, who at age 27 managed the volunteers department for World Youth Day in Krakow. Young Christians participating in World Youth Days are equipped with idealistic, yet real, goals: "to make the world a better place," she said.

Wasiutynska oversaw a team of almost 80 long-term volunteers and local team leaders and organized grounds for 19,000 short-term volunteers that came to Krakow from 63 countries around the world.

"I formed this team from scratch and dedicated three years of my life to organizing WYD," she said emphasizing that the outcomes are visible -- and long term.

"Being engaged in such an event gives you power and possibilities to later act locally. Many volunteers that came to 'our' WYD later engaged in their parishes. They still form groups and ministries, they embark themselves on foreign missions," she said.

Wasiutynska's work kept her busy, so when Stanislaw, a computer science engineer who had recently graduated from university, joined the committee two years before the event to develop the volunteer app, she was not interested in his friendly chats.

"I told Stanislaw I don't date until WYD is over. So he told me with a smile, 'I'll call two days after it's over.' And he did. I only learned later he fell in love with me the first day he saw me in the committee," she said. "He was really patient!"

They got married at the John Paul II Sanctuary in Krakow a year after WYD 2016 and now have three little girls, including 2-year-old twins.

World Youth Day "gave us foundations," Wasiutynska said. "It was a time of hard work, which prepared us for tough tasks both in professional and private lives. … Family life with three little kids is like managing a crisis 24/7. But the years we spent at WYD taught us that responsibility tastes best when taken courageously and complex free."

Asked how World Youth Day has impacted him, Yago de la Cierva laughed and asked, "You mean financially?" As a former executive director of WYD in Madrid in 2011, that is the question he received most often from journalists -- how expensive the event would be, or how much money the city would lose.

"They completely didn't get the point," de la Cierva said. "World Youth Day is a tangible example of the catholicity of the church: People from all around the world are there. And most of those coming haven’t experienced that before. That matters, not the money."

In 1985, he saw the church's global face at the International Youth Year event in Rome. In 1989, he accompanied a group of Roman university students to WYD in Santiago de Compostela, his hometown, as a guide.

"I saw the impact WYD had on them: being Roman, they thought of the church like something 'theirs,' and Santiago was the same shock (for them) it had on me four years earlier: to discover the universality of the church far away," he said.

In the Polish Marian shrine city of Czestochowa, the church's universality was visible in 1991 with what St. John Paul himself said the church breathes with "two lungs" -- one Eastern and one Western. "We were inspired by people of our age who had suffered a lot just for being Catholics under communism, and that was a healthy shock for lukewarm Catholics living in the West," de la Cierva said. "Our much poorer colleagues had a maturity and the moral strength that we Westerners simply didn’t have."

In 1997 in Paris, he witnessed a conversion of a volunteer named Marc, who said he had been baptized but had become an atheist, and whose World Youth Day experience ultimately led him back to the church.

Jarzembowski said World Youth Day is life-changing for so many because, as a pilgrimage, "it does break people down."

"I always advise group leaders if things are getting hard, stop and thank God for the hardness, because it's in the hardness that young people … grow," he said. "It challenges our comfort zone, it tests our limits. … You then are filled with grace. You're then filled with the Holy Spirit. You're filled with these messages of hope, of love, of faith. … You see a church bigger than yourself and so that then builds it up."

World Youth Days are also an antidote to the isolation and loneliness many young adults experience, Jarzembowski said. "It reminds you that you don't have to be alone."

Kate Fowler, 33, was a student at Santa Clara University in California and had recently returned from a semester abroad in Spain when she had the chance to go back for World Youth Day in Madrid. She was eager for more travel and excited to see the pope, but only had "a superficial understanding" of what to expect, she said.

"It took me going to realize how isolated and lonely I had felt as a young adult, as someone who had been going to Mass pretty faithfully every Sunday. I felt like, was religion, was faith, was Catholicism dying? Were there others like me who valued their faith?"

Fowler and other students traveled with two Jesuits to Spain ahead of World Youth Day for a "mini retreat" in Loyola, hometown of the Jesuits' founder St. Ignatius. They then spent a few days doing acts of service in Malaga, a port city on Spain's southern shore, before heading to Madrid for the main event.

"It was overwhelming in a beautiful way," Fowler said of World Youth Day. "For the first time in my life, I realized that the Catholic Church is so much bigger than my parish or my personal experience. I really saw that the body of Christ is universal, is alive and well, and that there were other young adults who were also committed and invested in their faith, talking about Jesus and meditating on Scripture."

Attending World Youth Day inspired Fowler to ask not what she wanted for her life, but what God wanted for her life, she said. She joined a prayer group, began praying the rosary and attending daily Mass and Eucharistic adoration. She later pursued a master's degree in theology and now works for a Washington-based Catholic apostolate, and is a wife and mother.

"There was such communion among people from all over the world," she said, "and the only thing that united everyone was Jesus Christ, and that was just incredible."