In consequence of the public calamities that afflicted the Diocese of Vienne in Dauphiny in the fifth century, St. Mamertus instituted a solemn penitential procession on the Monday, Tuesday and Wednesday before Ascension Day. Under an Order of the Council of Orleans in 511 the devotion spread to the rest of France. In 816 Pope Leo III introduced it in Rome and soon after it became a general observance throughout the Church.
The Litany of the Saints and the Psalms and Collects sung in procession on these days are supplications; hence the term Rogations applied to them. The object of these devotions is to appease the anger of God and avert the scourges of His justice and to draw down the blessings of God on the fruits of the earth.
Violet is used as a token of penance, and the Paschal candle is left un-lighted. The Litany of the Saints, consisting of ejaculations in the form of a dialogue, is an admirable manner of prayer which it should be our purpose to cultivate.
The Celebrant wears a violet stole and cope. All in the choir stand as they sing the Antiphon:
Exsurge, Domine, adjuva nos, et libera nos propter nomen tuum. * Deus auribus nostris audivimus, patres nostri annuntiaverunt nobis.
Arise, O Lord, help us and redeem us for Thy name's sake * We have heard, O God, with our ears: our fathers have told us.
Here all kneel, and two cantors begin the Litany of the Saints, the choir singing the responses. Each invocation must be repeated, except where it is found impossible to have the Procession. At Sancta Maria all stand and the procession begins to move, preceded by the processional cross and followed by the clergy, the celebrant and the faithful.
If a church or chapel is visited, the antiphon, versicle and Collect of the local patron saint may be sung, or the Stational Mass Exaudivit may be said. On leaving the interrupted Litany is resumed.
MASS OF ROGATION
Monday: at St. Mary Major.
Tuesday : at St. John Lateran.
Wednesday: at St. Peter's.
The Mass throughout points to the efficacy of the prayer of the just man when humble, sure and persistent. Elias by prayer closed and opened the heavens (Epistle), and our Lord shows us by two parables that God gives His Holy Spirit to whosoever asks Him, because He is good (Gospel, Alleluia). In our afflictions let us place our trust in God and He will hear our prayers (Introit, Collect).
The following Mass is said during or after the Procession of both the Greater (April 25th) and Lesser Litanies.
Exaudivit de templo sancto suo vocem meam, alleluia: et clamor meus in conspectu ejus, introivit in aures ejus, alleluia, alleluia. * Diligam te, Domine, virtus mea: Dominus firmamentum meum, et refugium meum, et liberator meus.
He heard my voice from His holy temple, alleluia; and my cry before Him came into His ears, alleluia, alleluia. * I will love Thee, O Lord, my strength; the Lord is my firmament, my refuge and my deliverer.
(Psalm 17:7,2-3 from the Introit of Mass)
Praesta, quaesumus, omnipotens Deus: ut, qui in afflictione nostra de tua pietate confidimus; contra adversa omnia, tua semper protectione muniamur.
Grant, we beseech Thee, O almighty God, that we who in our affliction confide in Thy mercy, may be ever defended by Thy protection against all adversity.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/13110b.htm