Known originally as In Dedicatione Basilicae S. Mariae ad Nives (Dedication of the Basilica of Saint Mary of the Snows), this feast day celebrates the story about the foundation of the basilica and Our Lady of the Snows.
The feast was originally inserted into the General Roman Calendar in 1568 after a request from the Council of Trent though the day had been celebrated for much longer in the Basilica and in most of the churches of Rome. It commemorates the dedication of the rebuilt Basilica di Santa Maria Maggiore by Pope Sixtus III. The original church, the Basilica Liberii was built during the pontificate of Pope Liberius.
The traditional founding of the Church goes back to the 4th century when a Roman patrician by the name of his John and his wife decided to donate their possessions to the Blessed Virgin Mary. The night they made their vow, the two gathered together and prayed asking her to show them how they should proceed in disposing of their property in her honor. Their answer came on August 5th when snow fell during the night on the summit of the Esquiline Hill. The snow made a clear spot on the top of the hill despite it being the middle of the Roman summer. As the snow fell, the two had a vision of the Blessed Virgin Mary directing them to build a basilica in honor of her on the spot covered in snow.
Over the following centuries many churches were build and dedicated to Our Lady of the Snows and 152 still exist today in Italy. Criticism of the legend began to build in the 18th century and a proposal sent to Pope Benedict XIV in 1741 asked him to have the legend be removed and the original name "Dedicatio Sancae Mariae" be restored. The wording remained intact however, for 228 years until the revision in 1969 in which the wording was changed to Dedicatio Sancae Mariae or the Dedication of the Basilica of Saint Mary Major with instructions to be celebrated as an Optional Memorial for August 5th.
On August 5th, at the conclusion of the Solemn Mass in the basilica, a shower of white rose petals is dropped from the dome of the Chapel of Our Lady. At sunset, an artificial snowfall is also staged right outside in the square of the basilica. There are several shrines of the Madonna della Neve, including the National Shrine of Our Lady of the Snows in Belleville, Illinois.