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In this case, the Feast of Our Lady of the Rosary commemorates the victory of Christian naval forces at the Battle of Lepanto on October 7, 1571. At a time when Christian Europe was being torn apart by internal strife and the Reformation, Don John of Austria destroyed the Turkish fleet in the Gulf of Lepanto. His victory was attributed to the intercession of the Blessed Virgin Mary, to whom rosaries were offered and processions were made in Rome on the day of the battle.
Source: http://catholicism.about.com/b/2007/10/07/our-lady-of-the-rosary.htm