Well if you missed the news in 2008.....

World Youth Day (or in short 'WYD') was initiated by Pope John Paul II in 1986. It is celebrated on a diocesan level annually, and at a week-long international level every two to three years at different locations. The international level events attract hundreds of thousands of youth from almost every country on the planet.

The next one will be held in Madrid, Spain from the 16-21st of August 2011.

Generic info on WYD can be found here http://en.wikipedia.org/wiki/World_Youth_Day