On many occasions throughout the course of history, Christian people have experienced the protection of the Virgin Mary. The title of Help of Christians, which Don Bosco associated with that of Mary Mother of the Church, is an indication of the special intervention of Mary in the most difficult trials of personal human life, of the Church and of the whole human family.

The liturgical celebration was instituted by Pope Pius VII in gratitude for Mary's intervention in a critical period in the history of the Church. Driven out by violence from the See of Rome and held as a prisoner for five years, the Pontiff had implored the help of Mary and had invited all Christians to do the same. Contrary to all expectations he was freed and returned to his See on 24 May 1814.

Devotion to Mary Help of Christians was spread far and wide through the work of St John Bosco, and continues to be propagated in the world by the Salesian Family, which recognizes and invokes Mary, Help of Christians as its principal patroness.

from: http://www.sdb.org/ENG/pdf/Maria_Colletta_Ufficio_en.pdf