The Name of the Feast of Our Lady of the Most Holy Rosary
The Feast of Our Lady of the Holy Rosary was known for two years, from 1571 to 1573, as The Feast of Our Lady of Victory. The victory that the title referred to was the victory won by Christian forces on October 7, 1571 when they defeated the naval power of the Ottoman Turks in a great sea battle at Lepanto.
The feast was renamed by Gregory XIII when he made his great reform to the Western calendar (which we now call Gregorian). It was Pius V, the pope at the time of Lepanto, who had instituted the initial feast. But why had he decided to honor Our Lady for victory in battle in the first place?
According to Dr. Kevin Johnson in his book Rosary, Pius attributed the victory to Mary because for several years he had "exhorted all the Faithful of Europe, including those Christians held in territories already conquered by the Turks, to pray the Rosary daily to beg Mary's intercession to save the Church and to liberate the captives."
To counter the progress that the Turks had made into Europe (by 1547 Hungary was part of the Turkish empire), the Pope encouraged Rosary processions in Rome. These large gatherings of Christians united in Marian prayer were copied throughout Europe, and a Rosary procession took place in Rome on the day the Battle of Lepanto was won. Johnson illustrates the fervor of their prayer: "Like the early Christian martyrs, like Christ himself in Gethsemane, the laity of Europe was praying for help against those who would kill them happily, and kill them horribly, in hatred of the Faith."
With the outpouring of Marian devotion in the face of danger from the Ottomans, it made sense, then, for Pius to trace back to the intercession of Mary a great military victory against them.
But it also made sense for Gregory to alter the title. He wanted to recognize the enduring importance of a particularly powerful devotional prayer: the Rosary. As Kevin Johnson explains: "the point of Lepanto wasn't so much Lepanto itself as the fact that it was through the Rosary that the destruction of Christendom had been averted." That particular military crisis had been resolved (temporarily, at least), but the Church would be faced with different kind of crises (the Reformation had raised serious questions about doctrine and discipline, for example).
The Rosary, the Church recognized then and recognizes now, is special way to overcome difficulties. Those difficulties may be geopolitical (as they were at Lepanto) or they may be personal (if we're struggling with our faith, with a temptation, or with acquiring some virtue). Whatever the magnitude of those difficulties, help can be found and victory won with the assistance of Mary through the Most Holy Rosary.