The introit should really begin before the priest even gets to the altar, although this is seldom observed. See the preface of the Liber Usualis. In most places the introit begins when the priest begins the prayers at the foot of the altar.

Yes, check out the rubrics in the Liber Usualis. Timing will also depend on logistics and customs in your church. In my experience for the 1962 Missal:

For Sunday Mass with Asperges rite, begin the Introit anytime after the Asperges rite ends (while the priest is vesting for Mass). Definitely during the prayers at the foot of the altar. For Masses without Asperges, the Introit can begin during the procession, or as soon as the priest reaches the sanctuary.

For the Kyrie, I begin anytime after the priest ascends the altar steps, so this chant accompanies the priest's recited Introit as well as the incensation, if that is taking place. The Gloria takes place after both chanted and recited Kyries are finished, so whoever finishes the Kyrie first (schola or priest) waits for the other before the Gloria is intoned.

Gradual/Alleluia. After the Epistle is finished. Often, the priest will lay his left hand on the altar as a signal to servers and schola.

Credo. When the priest returns to the center of the altar. Lately, I've learned to wait until he's replaced his maniple before giving the organ intonation.

The Offertory begins any time after the Dominus vobiscum.....Oremus.

Sanctus. Begin immediately after the bells are rung. Some churches separate Sanctus and Benedictus. In this case, I begin the Benedictus after the priest's genuflection to the Precious Blood.

Agnus Dei. After the Pax Domini.

Communio. This varies. Some places, the Communio is at the beginning of the congregation's Communion (just after Ecce Agnus Dei....Domine non sum dignus). Other places, the Communio is sung at the end, after the ablutions. I guess local custom holds here. Other Communion music can fit around, but stops when the priest goes to the Epistle side to recite the Communio and the post-communion prayer.

With the 1962 Mass, the schola is often singing while other actions are taking place in the sanctuary--I think of this as liturgical multi-tasking.

I have heard arguments on exactly what may be sung of the High Mass at the Low Mass. One argument is that nothing whatsoever of the High Mass may be used at a Low Mass. The other one was that any of the Propers can be used, but as soon as anything from the Ordinary is sung, that make it a High Mass and everything MUST be sung. The Ordinary does involve the Celebrant and the people, so it's logical that a High Mass would be happening. At a Low Mass, all of the music is simply 'music performed AT the Low Mass', it is not the Mass. In that case, the music should never impede the flow of the Mass - which the priest is saying. Many parishes have the priest reading the Lessons aloud, in a clear voice, so the people can hear every word of the Latin. With translations in their hands, then there is no need for the Lessons to be repeated at the beginning of the Sermon. I think this is a beautiful and important part of the Low Mass. I'm involved in one weekly Low Mass where the whole congregation says the Prayers and the foot of the Altar, and I actually like that, too.

I have been singing Introits as preludes for some time now. Almost all of them take between 4 and 5 minutes. If you start as soon as the Entrance into the Sanctuary (hence the name "Introit") it should take you right through to the priest starting the Introit or Kyrie. Offerories (except some during Lent that are quite long) can easily fit their liturgical space. And it is permissible to add Psalm verses to the Communion Antiphon to fill the whole time there. If the priest is willing to let you take the extra time between the Lessons, you might consider using the Rossini 'Psalm tone' versions of those parts to save time, and to learn the tones.

If you are looking for organ accompaniment to any of this, send me an email privately.

Michael O'Connor,

I'm glad to hear your schola's first Mass went well, and that the priest sang his parts. Though not the ideal, the priest can sing his parts recto tono (on one note) in a high Mass, comparable to a beginning schola using psalm tone or Rossini for propers. You might encourage your priest to sing recto tono, especially if he doesn't know the chants or doesn't consider himself very musically talented.