The Title Our Lady of the Rosary can be traced back to 1571 and the Battle of Lepanto.
The Battle of Lepanto was a large-scale naval battle fought between a coalition of Catholic nations known as the Holy League and the Islamic Ottoman Empire. The Holy League was in command of 168 vessels while the Ottomans were in command of 337 vessels. By all means, the Ottomans had the advantage. Knowing the Holy League was at a disadvantage, Pope Pius V called for all of Europe to pray the Rosary for their victory.
At the beginning of the battle, the wind threatened to prevent the Holy League from forming a line to hold off the oncoming Ottoman ships. By some miracle, however, the wind shifted shortly before the two sides made contact allowing the Holy League to get into position. By the end of the battle, the Holy League had captured 137 vessels, killed 30,000 Turks, and took over 10,000 prisoners while only suffering 7,500 deaths. Despite all odds, it was a major victory for the Holy League.
In thanksgiving for this decisive victory, St. Pius V instituted the feast of Our Lady of Victory to be celebrated on the day of the victory, October 7th. Two years later, Pope Gregory XIII changed the name of the feast day to the Feast of the Holy Rosary, leading to the title Our Lady of the Holy Rosary.