Our Lady Help of Christians.
This feast was instituted by Pope Pius VII in 1815, when the exile of the Supreme Pontiff, due to the French Revolution, was brought to an end.
Omnipotens et misericors Deus, qui ad defensionem populi Christiani in beatissima Virgine Maria perpetuum auxilium mirabiliter constituisti: concede propitius; ut tali praesidio muniti, certantes in vita, victoriam de hoste maligno consequi valeamus in morte.
Almighty and merciful God, who in the most blessed Virgin Mary hast marvellously given us ever present help for the defence of the Christian people; grant, in thy mercyl, that in life's battle we may be shielded by her powerful protection and at death may be able to vanquish the wicked enemy.
(Collect at Mass)
See the Catholic Encyclopaedia for more information on this Feast: http://www.newadvent.org/cathen/11360c.htm