My wife and I dropped by Corpus Christi parish in Celebration, Florida for Mass today and encountered something wonderful. The Communion hymn was The Litany of the Saints. You hear this all the time at ordinations and during the Easter Vigil, but this was something really unexpected. It was beautifully sung— and brought back memories of my own ordination, when I heard it while lying face down on the floor of a basilica, with tears streaming down my cheeks. (I confess to choking back some tears this afternoon when I heard it, too.)
The Litany of Saints is also prescribed for ordination (different saints are added corresponding to the different grades of ordained ministry), religious profession, the blessing of an abbot, and the dedication of churches and altars.
In the Latin language version of the Litany, the names of one or more saints are chanted by a cantor or choir, and the congregants reply with either, Ora pro nobis (if one saint is addressed) or Orate pro nobis (using the plural imperative form of the verb, if more than one saint is addressed). Both responses translate to “Pray for us.” However, it is permissible to personalize the Litany of Saints for a funeral rite or other Mass for the dead. When this was done during the Funeral of Pope John Paul II and recently the Funeral of Pope Emeritus Benedict XVI, the response was Ora[te] pro eo, or “Pray for him.”
A Vatican recommendation issued in 1988 proposes that the Litany can be appropriately used for the beginning of the Mass of the First Sunday of Lent, to offer a distinguishing mark for the beginning of Lent.
You can find more at the link on who is included, and in what order.
For All Saints Day: All you holy men and women, pray for us!