From The Catholic Herald:
White rose petals fell from the ceiling of St. Mary Major Basilica Monday, as Romans celebrated the anniversary of a 4th century Marian miracle.
The miracle, which inspired the construction of the papal Marian basilica, involved a miraculous snowfall in Rome on August 5 in the year 358. According to tradition, the Virgin Mary appeared to both a nobleman named John and to Pope Liberius (352-366) in a dream foretelling the August snow and asking for a church to be built in her honor on the site of the snowfall. The church was rebuilt by Pope Sixtus III (432-440), after the Council of Ephesus in 431 declared Mary to be the Mother of God.