It is not used in the Novus Ordo, though it was brought out of retirement for the Jubilee. If you use the 1962 books, however, it is there.
It consists of a sung antiphon followed by a sung collect: the antiphon is begun by a cantor and taken up by everyone, and the celebrant sings the collect. Or it can all be said, if the Mass was a Low Mass.
I encourage all priests who celebrate the Traditional Mass to use this, particularly this Sunday. What Mass is the 'principle Mass on Sunday', once something quite clear-cut, has become rather subjective. But if it is the principle Mass, it should be preceded by another beautiful ceremony, the Asperges. (Again, it is perfectly possible to do this at Low Mass, without singing it.)
V. Dómine salvum fac Regem nostrum Cárolum.
R. Et exáudi nos in die, qua invocavérimus te.
Quǽsumus, omnípotens Deus, ut fámulus tuus Cárolus, Rex noster, quæ tua miseratióne suscépit regni gubernácula, virtútum étiam ómnium percípiat increméntum; quibus decénter ornátus et vitiórum monstra devitáre, [in time of war: hostes superáre,] et ad te qui via, véritas, et vita es, cum regina consorte et prole régia gratiósus valeat perveníre. Per Christum Dóminum nostrum.
V. O Lord, save Charles our King
All: and hear us in the day that we shall call upon thee.
Let us pray.
We beseech thee, almighty God, that thy servant Charles our King, who through thy mercy has undertaken the government of this realm, may also receive an increase of all virtues. Fittingly adorned with these, may he be able to shun all evildoing, [in time of war: to vanquish his enemies,] and, together with the Queen consort and the royal family, to come by thy grace unto thee who art the way, the truth, and the life. Through Christ our Lord.
This is a jpeg of three different Chant settings of the antiphon: there are also polyponic settings. The first given here is the most commonly used one.
Nothing in life being simple, the following version of the first Chant setting seems to be a better understanding of how to fit in the name 'Carolus' into the melody.
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