Since 1884, October has been considered the month of the Rosary. The tenth month of the year was dedicated to the Rosary by Pope Leo XII through a short encyclical, Superiore Anno, which means "last year."
The document requests that "from the 1st of October to the 2nd of November following in all the parish churches (curialibus templis), in all public churches dedicated to the Mother of God, or in such as are appointed by the Ordinary, five decades at least of the Rosary be recited, together with the Litany..."
He wrote that a Plenary Indulgence can be received by Catholics who have performed the prayers described above ten times during the thirty-three-day period and are in a state of grace.
The presence of a special dedication to the Rosary in October began over 450 years ago, in 1571. In the mid-1500s, the Ottoman Empire was spreading west, with the eventual goal of conquering all of Europe, bringing Islam with it. By the end of the 1560s, European Catholic powers knew they needed to take action. The Holy League, an alliance, was founded. On October 7th, 1571, over 200 of their ships set sail to confront the Ottomans near Greece.
In preparation for the battle, the soldiers prayed the Rosary together as Pope Saint Pius V requested. The Pope also asked all the entire Church in Europe to gather and pray the Rosary in advance of the battle. Though outnumbered, the Holy League was victorious, preventing the Ottoman Empire, and Islam, from spreading west.
The victory was credited to the intercession of the Blessed Virgin Mary. In 1573, the Feast of Our Lady of the Rosary was instituted to take place annually on October 7th by Pope Gregory XIII.
Though the Rosary is a prayer precious to Catholics year-round, the Church holds special devotion to it this month; it is seen as pivotal to preserving Christendom in Europe.