The month of October is dedicated to the Holy Rosary — by personal recommendation of Pope Leo XIII:
In a letter of September 1, 1883, mindful of the Rosary’s power to strengthen faith and foster a life of virtue, he outlined the triumphs of the Rosary in past times and admonished the faithful to dedicate the month of October to the Blessed Virgin through the daily recitation of her Rosary in the presence of the Blessed Sacrament, in order to obtain through her intercession the grace that God would console and defend His Church in her sufferings.
Beginning on September 1, 1883, with SUPREMO APOSTOLATUS OFFICIO, Pope Leo wrote a total of eleven encyclicals on the Rosary, ending with DIUTURNI TEMPORIS in 1898. (Source: Rev. Matthew R. Mauriello, Catholic.net).
The spread of the devotion of the rosary is attributed to the revelation of Mary to St. Dominic, who sought her help in battling the heresy of the Albigenses. Robert Feeney’s “St. Dominic and the Rosary” gives a detailed account,