Why on the 12th of September?
The feast originated in Spain and was approved by the Holy See in 1513; Innocent XI extended its observance to the whole Church in 1683 in thanksgiving to our Lady for the victory on September 12, 1683 by John Sobieski, king of Poland, over the Turks, who were besieging Vienna and threatening the West. This day was commemorated in Vienna by creating a new kind of pastry and shaping it in the form of the Turkish half-moon. It was eaten along with coffee which was part of the booty from the Turks.
(from Mariology by Juniper B. Carol, O.F.M., courtesy of www.catholicculture.org)
Behold the quintessential Vatican breakfast: