This year marks the 10th anniversary of the last World Youth Day attended by Pope John Paul II, which happened in Toronto, Canada.
The theme of the 2002 event was "You are the salt of the earth ... you are the light of the world", and it brought nearly half a million people to the city.
After a decade, the priest most involved with bringing WYD to Canada reflected on its significance.
"I've always said that World Youth Day is a timed-released capsule," said Father Thomas Rosica, C.S.B., the National Director of the Toronto World Youth Day. "Anybody who waits for immediate results from a World Youth Day in the six-months afterwards, or are expecting the churches are going to be populated immediately will be terribly let down."
Father Rosica said WYD reinvigorates the Church in ways that become apparent over time.
"We look for the deeper signs of the effect of World Youth Day," he told Vatican Radio. "That is a reinvigorated young adult ministry, pastoral ministry for young people, university chaplaincies, renewed vocations: An awareness that the Church is alive and the Church is young, as Pope Benedict said so beautifully in his inauguration."
One other concrete result was the establishment of Salt + Light Television, Canada's first national Catholic television network.
After the success of World Youth Day, Father Rosica was also asked to take charge of this new enterprise.
"I thought my work was done after World Youth Day. I wanted to go away, hibernate, and take a long sabbatical," he said.
"The more I look at it now in hindsight and also with the eyes of faith, retrospection, and providence, it was God's way of saying the Church is alive and the Church is young, because the spirit of World Youth Day has continued in Salt + Light," he told Vatican Radio.
The next World Youth Day will be in 2013 in Rio de Janeiro.