Why do we celebrate Our Lady of the Rosary?

Pope St. Pius V knew that Our Lady had interceded for the Holy League, ensuring their victory. In fact, the day of the Battle of Lepanto, October 7, 1571, he saw the Blessed Mother in a vision, and she revealed the victory to him.

As an offering of thanksgiving, the Pope declared this day as the feast day of Our Lady of Victory, later known as Our Lady of the Rosary.