World Youth Days (WYDs) are international meetings of young people organized by the Catholic Church every two or three years. They are held in a different city each time. Launched in 1985, they were an important component of Pope John Paul II’s "New Evangelisation". They were then continued by the next popes, Benedict XVI and Francis. The most recent festival took place in Panama in 2019. Studying these great itinerant and global celebrations as strategic devices of religious education and socialization shows how a denominational institution, the Catholic Church, has sought to adapt to the end of inherited religious identities, caused by the "failure" in the traditional vehicles of transmission that were family and school. After presenting the context in which WYDs emerged, this article analyses the means used by their organizers to "transmit religion" before evaluating their impact in the short and medium term. The survey draws on the existing scientific literature on the subject, interviews with former protagonists and information gathered from ecclesiastical archives in Rome, Paris, Ottawa, Quebec City, and Manila.