"Our Lady of the Snows": The story behind the "major", or greatest, Basilica dedicated to Our Lady in Rome
When Liberius was Pope, a Roman patrician named John, and his wife, also of noble birth, having no children to inherit their goods, vowed their inheritance to the most holy Virgin Mother of God. The blessed Virgin heard their prayers and approved their vow by a miracle. On the 5th of August, which is that time when the heat of summer waxeth greatest in the City, a part of the Esquiline Hill was covered by night with snow. And on that same night, the Mother of God told John and his wife separately in dreams that they should build a church on that place. When John told this to Pope Liberius, he said that he had had the same dream. The Pope therefore went to the snow-covered hill and there marked out a site. The church was built with the money given by John and his wife, and was later restored by Sixtus III. It hath been given various names; but, so that its title may indicate its excellence, it is called the Church of St. Mary Major.
From the Breviarium Romanum