October 7 is the anniversary of the glorious victory won in 1571 by the Christian forces over the Turkish fleet in Lepanto. This triumph of the Cross over the Crescent was universally attributed to the powerful intercession of the Mother of God. Whom Pope Pius V fervently invoked with her Rosary in his hand, and to whom the prayers of all Christendom were addressed. Two years after this great favor had been obtained Gregory XIII instituted an annual feast of thanksgiving to be celebrated on the First Saturday of October in all churches where an altar in honor of the Blessed Virgin of the Rosary had been erected. From Being a local festival this celebration gradually spread and became general, until Leo XIII raised it to the rank of a double of the second class for the whole Chruch.

The devotion of the Rosary of the Blessed Virgin Mary dates at least from the Twelfth Century. The glory of having spread this form of prayer with such extraordinary success is certainly due to the Dominican Order, and, owing to their zeal, the Rosary soon became the most popular devotion throughout the Christian world.
The Holy Rosary as it is now recited, and enriched with great indulgences, represents, after the Divine Office, what may be described as a popular Breviary of the Gospel. By meditating on the appropriate mysteries, it may be adapted to the Liturgical Cycle, and because it unites vocal with mental prayer the Rosary is regarded as the most beautiful and approved devotion of the Latin Church.

(Source: The New Roman Missal, Rev. F. X. Lasance)