It was Pope Leo XIII who named October as the month of the rosary, and today is the Feast of Our Lady of the Rosary. How it got started is an interesting story. In 1571, as the forces of the Ottoman Empire threatened western Europe, Pope St. Pius V persuaded the rulers of Spain, Genoa and Austria to join their forces together in a Holy League against the Turks. On this day in 1571, under the leadership of Don Juan of Austria, they defeated the Turkish naval forces in the Battle of Lepanto. This victory was attributed to the intercession of the Blessed Virgin Mary, as rosary processions had been organized to pray for a victory. In 1573, Pope Pius instituted this feast as a thanksgiving for the victory over the Turks. In 1716, Pope Clement XI extended the feast to the universal Church. G.K. Chesterton wrote a poem about the battle, which you can read here.