Leo XIII, in his encyclicals, has consecrated the present month to this devotion so dear to heaven; he has honored our Lady in her litanies with a new title, Queen of the Most Holy Rosary, and he has given the final development to the solemnity of this day, by raising it to the rank of a second class feast, and by enriching it with a proper Office explaining its permanent object. Besides all this, the feast is a memorial of glorious victories, which do honor to the Christian name.
Soliman II, the greatest of the Sultans, taking advantage of the confusion caused in the west by Luther, had filled the sixteenth century with terror by his exploits. He left to his son, Selim II, the prospect of being able at length to carry out the ambition of his race: to subjugate Borne and Vienna, the Pope and the emperor, to the power of the crescent. The Turkish fleet had already mastered the greater part of the Mediterranean, and was threatening Italy, when, on October 7, 1571, it came into action, in the Gulf of Lepanto, with the pontifical galleys supported by the fleets of Spain and Venice. It was Sunday; throughout the world the confraternities of the Rosary were engaged in their work of intercession. Supernaturally enlightened, St. Pius V watched from the Vatican the battle undertaken by the leader he had chosen, Don John of Austria, against the three hundred vessels of Islam. The illustrious Pontiff, whose life’s work was now completed, did not survive to celebrate the anniversary of the triumph; but he perpetuated the memory of it by an annual commemoration of Our Lady of Victory. His successor, Gregory XIII, altered this title to Our Lady of the Rosary, and appointed the first Sunday of October for the new feast, authorizing its celebration in those churches which possessed an altar under that invocation.
— The Liturgical Year Volume 14: The Time After Pentecost, Volume 5, by Dom Prosper Guéranger OSB, p. 297.