235. The Litany of the Saints has been used in the Roman Church since the seventh century (Cf. Ordo Romanus in A. Andrieu (ed.), Les “Ordines Romani” du Haut Moyen-Age, III, Spicilegium Sacrum Lovaniense, Lovain 1951, p. 249. For indulgences cf. EI, Aliae concessiones, 22, p. 68). Its liturgical structure is subtle, simple and popular. Through the litany, the Church invokes the Saints on certain great sacramental occasions and on other occasions when her imploration is intensified: at the Easter vigil, before blessing the Baptismal font; in the celebration of the Sacrament of Baptism; in conferring Sacred Orders of the episcopate, priesthood and deaconate; in the rite for the consecration of virgins and of religious profession; in the rite of dedication of a church and consecration of an altar; at rogation; at the station Masses and penitential processions; when casting out the Devil during the rite of exorcism; and in entrusting the dying to the mercy of God.
The limitation in many of these circumstances, including at a parish’s Easter Vigil, is the addition of important patron saints of importance to the local community: parish patron, founder(s), etc.. Names of blesseds and saints may be included, if they have received formal Church recognition:
The Litanies of the Saints contain elements deriving from both the liturgical tradition and from popular piety. They are expressions of the Church’s confidence in the intercession of the Saints and an experience of the communion between the Church of the heavenly Jerusalem and the Church on her earthly pilgrim journey. The names of the Beati that have been inscribed in the calendars of particular Churches or religious institutes may be invoked in the litanies of the Saints (Cf. CDWDS, Notificatio de cultu Beatorum,13, in Notitiae 35 (1999) 446). Clearly, the names of those whose cult has not received ecclesial recognition should not be used in the litanies.
The Directory on Popular Piety and the Liturgy is online at the Vatican site.