World Youth Day has origins in the International Jubilee of Youth of 1984 in Rome. Pope John Paul II had opened the Holy Year of Redemption in 1983 to commemorate the 1,950th anniversary of the death of Jesus Christ, which affected the redemption of the human race. He asked the young people of the world to meet him in Rome the following year, and they did, gathering in the Eternal City on Palm Sunday, April 15, 1984.
Three hundred thousand youths from across the globe came to Rome, and joining them was an elderly nun from Calcutta, Blessed Mother Theresa. The following year (1985) was declared International Year of the Youth by the United Nations, so the pope invited the kids back to Rome for another international gathering and three hundred thousand showed up as in the previous year. The actual first time the phrase World Youth Day was coined, however, was in 1985, when John Paul announced that there would be a World Youth Day in Buenos Aires, Argentina, on April 11 and 12, 1987. The year before, on Palm Sunday of 1986, many local dioceses had gatherings for youth and gave Pope John Paul II a one-year breather, so to speak.
1987 was the first official international World Youth Day held outside of Rome, in Argentina. Pope John Paul traveled to Buenos Aires for the event with 900,000 people in attendance. Thus began the tradition of having one international World Youth Day every two to three years for which he would travel to a foreign land, with a smaller local gathering in Rome on the intermittent years to allow the bishops of the world to have their own celebrations as well.
During the few days of World Youth Day, the Pope speaks specifically to the youth; this event is not considered a pilgrimage or a state visit to national leaders. World Youth Day is for young people, and it is always distinct and separate from papal visits to countries.
1989: Santiago de Compostela, Spain; 400,000 attended
1991: Czestochowa, Poland; over 1.5 million attended
1993: Denver, CO, USA; 500,000 attended
1995: Manila, Philippines; 5 million attended
1997: Paris, France; 1.2 million attended
2000: Rome, Italy; 2 million attended
2002: Toronto, Canada (the last WYD for John Paul); 800,000 attended
2005: Cologne, Germany (the first time with Benedict XVI ); 1.2 million attended
2007: Scheduled for Sydney, Australia