The title "Our Lady Help of Christians" was first introduced after the Battle of Lepanto. Pope St. Pius V commanded the faithful to pray the rosary for victory, and their prayers turned the tide as Our Lady led the inferior Christian fleet to soundly defeat the Muslim Turks. This great battle occurred on October 7th 1571, for which reason this date has been chosen as the Feast of the Holy Rosary. Pius V inserted the title "Help of Christians" in the Litany of Loreto in 1571 in thanksgiving to Our Lady.
Near the end of the 17th century, Emperor Leopold I of Austria took refuge in the Shrine of Mary Help of Christians at Pasau, when the Ottoman Turks besieged the capital city of Vienna. Again a great victory occurred through the intercession of Mary Help of Christians when, on September 12th, the Feast of the Holy Name of Mary, Vienna was finally freed.
The feast day itself was originated in 1815 by Pope Pius VII. The Pope, while undergoing severe trials and difficulties under the reign of Napoleon, often invoked the help of Our Lady. While he was in exile in the Italian city of Savona, he encouraged the faithful there to join him in a Rosary Novena to Our Lady Help of Christians for the intention of his release.
Our Lady answered their prayers and the Pope was able to return to the Papal States. However, upon his return to Rome, Napoleon's men arrested him and imprisoned him for another five years, first in Grenoble and then in Fontainbleau. During this time the Pope prayed to Our Lady and also made a solemn promise to God that should he be liberated and be able to return to the Papal States, he would institute a special feast day in honor of Our Lady.
On May 24, 1814 the Pope was released from his imprisonment and returned to Rome. As promised, he instituted a feast day in honor of Our Lady Help of Christians, celebrated on May 24th.