Here's the Report from News.va:
After his arrival at Ciampino airport, the Holy Father chose to stop at the Basilica of Saint Mary Major to offer a brief prayer to the Blessed Virgin for World Youth Day.
Seeing the Pope as he entered the Basilica, a group of young people approached him and offered him a T-shirt and a ball. Pope Francis later offered the gifts to the Madonna.
This is one of the great churches of Christendom, and recalls a time when it snowed in the summer as a sign of Our Lady's favor.
It was commemorated by the command of St. Pius V in connection with Trent. Taken from wikipedia's article:
"During the pontificate of Liberius, the Roman patrician John and his wife, who were without heirs, made a vow to donate their possessions to the Virgin Mary. They prayed that she might make known to them how they were to dispose of their property in her honour. On 5 August, at the height of the Roman summer, snow fell during the night on the summit of the Esquiline Hill. In obedience to a vision of the Virgin Mary which they had the same night, the couple built a basilica in honour of Mary on the very spot which was covered with snow."