There has been a slight change made in this ceremony–the priest no longer says the Miserere while he sprinkles the people (see “Ephemerides Liturgicae” 75  426), and the wording of the rubric for Passiontime and Eastertime also has been altered. The significance of this blessing is touched on in the commentary given above, and the frequent omission of this blessing is noted with regret. Some say that it interferes with the introit procession, but some solution could be found.
The priest who is to offer the Mass, vested in cope of the proper color, comes to the altar, and as he kneels on the step with the ministrants (also in Eastertime) he receives the aspersory from the deacon. First he sprinkles the altar three times (simultaneously intoning the antiphon), then himself, and then he stands and sprinkles the ministrants. The choir takes up the singing of the antiphon, during which time the celebrant sprinkles the clergy and the people. The proper antiphons are given below (for the music for these see the music supplement).
Antiphon outside Eastertime
Purify me with hyssop, * Lord, and I shall be clean of sin. Wash me, and I shall be whiter than snow. Ps. 50.1. Have mercy on me, God, * in your great kindness. V. Glory be to the Father, and to the Son, and to the Holy Spirit. * As it was in the beginning, is now, and ever shall be, world without end. Amen. Purify me with hyssop, Lord, and I shall be clean of sin. Wash me, and I shall be whiter than snow.
The antiphon is sung thus at the sprinkling with holy water on all Sundays outside Eastertime; but the doxology is not said during Passiontime, and the antiphon is repeated right after the psalm verse.
During Eastertime, from Easter Sunday until Pentecost inclusive the following antiphon is sung:
Antiphon during Eastertime
I saw water * flowing out from beneath the threshold of the temple, alleluia; and all to whom this water came were saved, and they shall say, alleluia, alleluia. Ps. 117. Give thanks to the Lord, for He is good, * for His mercy endures forever. V. Glory be to the Father, and to the Son, and to the Holy Spirit. * As it was in the beginning, is now, and ever shall be, world without end. Amen. I saw water flowing out from beneath the threshold of the temple, alleluia; and all to whom this water came were saved, and they shall say, alleluia, alleluia.
The first antiphon given above is resumed on Trinity Sunday.
On Easter Sunday, in churches where there is a baptismal font, the water used for the sprinkling is that which has been blessed during the Easter Vigil, that which was taken from the font before the holy oils were poured in.
After the singing of the antiphon the priest, who by this time has returned to the altar, stands at the foot of the altar, and with hands folded chants the following:
P: Lord, show us your mercy (alleluia).
All: And grant us your salvation (alleluia).
P: Lord, heed my prayer.
All: And let my cry be heard by you.
P: The Lord be with you.
All: May He also be with you.
Let us pray.
Hear us, holy Lord and Father, almighty everlasting God, and in your goodness send your holy angel from heaven to watch over and protect all who are assembled in this dwelling, to be with them and give them comfort and encouragement; through Christ our Lord.