This is the rite of sprinkling of the congregation with Holy Water at the beginning of the Sunday mass, while an antiphon (and psalm) are sung, typically by the choir.
There are two different antiphons used during this rite:
Vidi Aquam for Eastertide, from Easter Sunday until Pentecost
Asperges Me for outside of Eastertide.
For this rite, the celebrant vests in a cope before mass, and his chasuble is laid out in the chancel.
The aspersorium and aspergillium are placed in the chancel on a side table.
At the entrance, upon arriving at the altar step, the celebrant and deacon genuflect and immediately take up the aspersorium and aspergillium.
The celebrant first sprinkles the altar (three times), then blesses himself and blesses the other ministers and servers. He then turns (either alone or with the deacon) and moves through the nave blessing the congregation.
Servers go there places in the chancel pews as usual.
All make the sign of the cross as they are blessed.
During the sprinkling the choir sings the appropriate antiphon and psalm. All bow at the Glory be.
Note that the Glory be is omitted on Passion Sunday and Palm Sunday.
Upon returning to the altar step, the celebrant says the prayer, as the servers and congregation make the appropriate responses.
He then changes from cope to chasuble with the assistance of the deacon and acolyte. The MC removes the cope to the sacristy.
Mass continues as normal with all standing at the foot of the altar for the Introit.
Rite of Sprinkling Holy Water: Asperges or Vidi Aquam
At the beginning of the principal Sunday Mass, the Priest may sprinkle the People with holy water. Having blessed the water, the Priest, vested in a cope of the colour of the day, proceeds to the altar. And then, kneeling with the other sacred Ministers as the steps, he receives the aspergillum from one of the Ministers or servers, and first sprinkles the altar three times, then he blesses himself and, standing up, blesses the Ministers and servers as he or the cantor begins the antiphon. The choir continues the antiphon and the corresponding psalm, while the Priest sprinkles the People.
Outside Eastertide: Asperges me
From Easter Sunday until Pentecost: Vidi aquam
Having sprinkled the People, the Priest returns to the foot of the altar and with hands joined continues:
Priest: O Lord, show thy mercy upon us (Alleluia)
People: And grant us thy salvation (Alleluia)
Priest: O Lord, hear my prayer.
People: And let me cry come unto thee.
Priest: The Lord be with you.
People: And with thy spirit.
Priest: Let us pray.
O Lord , holy Father, Almighty, everlasting God, we beseech thee to hear us; and vouchsafe to send thy holy Angel from heaven, to guard and cherish, protect and visit, and evermore defend all who are assembled in this place; through Christ our Lord. Amen.
The Priest removes the cope and assumes the Mass vestments, and the Introit is sung. The Priest ascends to the altar and venerates it with a kiss and, if appropriate to the dignity of the celebration, he censes the altar and cross in teh customary manner. Mass continues with the Collect for Purity.