The following comes from the CNA:
The Feast of Mary Help of Christians is celebrated on May 24.
The tradition of this advocation goes back to 1571, when the whole of Christendom was saved by Mary Help of Christians when Catholics throughout Europe prayed the Rosary. The great battle of Lepanto occurred on October 7th 1571. For this reason this date has been chosen as the feast of the Holy Rosary. In 1573 Pope Pius V instituted the feast in thanksgiving for the decisive victory of Christianity over Islamism.
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 200,000 Ottoman Turks besieged the capital city of Vienna, but a great victory occurred thanks to Mary Help of Christians: on September 8th, Feast of Our Lady's Birthday, plans were drawn for the battle. On September 12, Feast of the Holy Name of Mary, Vienna was finally freed through the intercession of Mary Help of Christians. All Europe had joined with the Emperor crying out "Mary, Help!" and praying the Holy rosary.
In 1809, Napoleon's men entered the Vatican, arrested Pius VII and brought him in chains to Grenoble, and eventually Fontainbleau. His imprisonment lasted five years. The Holy Father vowed to God that , if he were restored to the Roman See, he would institute a special feast in honor of Mary. Military reverses forced Napoleon to release the Pope, and on May 24th 1814, Pius VII returned in triumph to Rome. Twelve months later, the Pope decreed that the feast of Mary Help of Christians, be kept on the 24th of May.
St. John Bosco (1815 - 1888) was a dynamic priest who founded the Salesian Order in the XIX century in Italy. His many prophetic dreams, beginning at age nine, guided his ministry and gave insights on future events.
On May 14, 1862, Don Bosco dreamed about the battles the Church would face in the latter days. In his dream, the Pope of those days anchors the 'ship' of the Church between two pillars, one with a statue of Mary (Auxilium Christianorum or 'Help of Christians') and the other with a large Eucharistic Host
St. John Bosco wrote about his congregation, the Salesians: "The principal objective is to promote veneration of the Blessed Sacrament and devotion to Mary Help of Christians. This title seems to please the august Queen of Heaven very much."
The Salesian Sisters of St John Bosco or Daughters of Mary Help of Christians, are the sister order of the Salesians of Don Bosco.
St. John Bosco, himself, on June 9 1868, dedicated to Our Lady Help of Christians, the mother church of his congregation at Turin (Italy). The Salesian Fathers and their Sisters have carried the devotion to their numerous establishments.