The rosary is a beautiful meditative prayer on the life of Christ and how he won our salvation. In October, we celebrate the feast of Our Lady of the Rosary and dedicate the entire month to Mary under this title. This tradition began in 1571 when Pope Pius V decreed October 7 as a feast day to celebrate victory over the Turks in the Battle of Lepanto. In 1573, he began the Feast of the Holy Rosary, which was celebrated at those churches that had a Rosary Altar. This remained in place until Pope Clement XI extended the feast to the entire Church in 1716. Pope Pius X decided to have the Church celebrate the feast on October 7. This did not become a mandatory memorial though until 1969, after Vatican II.
The rosary was not, as legend tell us, given to St. Dominic by Mary. The first rosary appeared in the late Middle Ages as a substitute for lay monks and devout lay people who wished to pray the Divine Office but could read the psalms. Instead of the psalms, 150 Our Father’s were said. By the 12th century, people began substituting Hail Mary’s for the Our Father’s.
During the 13th century, Dominican friars, divided the rosary into 15 decades and then the three sets of mysteries; Joyful, Sorrowful and Glorious. These mysteries, based in scripture, tell us the history of our salvation. In 1569 Pope Pius V approved the rosary in its’ current form. In 2002, Pope John Paul II, again drawing from scriptures, added the Luminous (light) mysteries.
Information taken from: http://campus.udayton.edu/mary/resources/rosdex.html