The Basilica of Santa Maria Maggiore is the most important and best preserved of Roman churches dedicated to the Madonna. It rises on top of the Esquiline Hill and is the only remaining example of early Christian architecture, even though it has over time been further embellished.

Work on the current basilica, which was built on top of an earlier church, was embarked on thanks to a wealthy Roman patrician called Giovanni.
According to legend, Giovanni and his wife, who were without child, dedicated a church to the Virgin Mary who had appeared to them in a dream on the night of 4th/ 5th August 352 A.D.
The Madonna had told them that a miracle would show them the location for them to build the church. Pope Liberius also had the same dream and the following day on arriving on the Esquiline Hill,  found it had indeed been miraculously covered in snow.. in the heat of August. Liberius then traced an outline of the location of the planned church, the building of which was financed by the married couple.

This then explains why the basilica is also referred to as Our Lady of the Snow and how, every year on 5th August, the miracle of the snow is commemorated with a cascade of white petals descending from the coffered ceiling onto the altar place, an evocative sight really not to be missed.
http://www.turismoroma.it/cosa-fare/basilica-di-santa-maria-maggiore?lang=en