200. Visiting Priests should be gladly admitted to concelebration of the Eucharist, provided their Priestly standing has been ascertained.
I’ve always wondered how priests present their credentials when traveling. As the parish liturgist, I’m usually introduced to the visiting priest. I’ve never been asked to validate his good standing.
201. When there is a large number of Priests, concelebration may take place even several times on the same day, where necessity or pastoral advantage commend it. However, this must be done at different times or in distinct sacred places.[Eucharisticum Mysterium 47]
202. It is for the Bishop, in accordance with the norm of law, to regulate the discipline for concelebration in all churches and oratories of his diocese.
Bishops govern this practice locally, of course.
203. To be held in particularly high regard is that concelebration in which the Priests of any given diocese concelebrate with their own Bishop at a stational Mass, especially on the more solemn days of the liturgical year, at the Ordination Mass of a new Bishop of the diocese or of his Coadjutor or Auxiliary, at the Chrism Mass, at the Evening Mass of the Lord’s Supper, at celebrations of the Founder Saint of a local Church or the Patron of the diocese, on anniversaries of the Bishop, and, lastly, on the occasion of a Synod or a pastoral visitation.
In the same way, concelebration is recommended whenever Priests gather together with their own Bishop whether on the occasion of a retreat or at any other gathering. In these cases the sign of the unity of the Priesthood and also of the Church inherent in every concelebration is made more clearly manifest.[Eucharisticum Mysterium 47]
I still recall a very moving experience from my grad school days. I was invited to assist with music for a liturgy at the conclusion of a priest retreat in my diocese. The attention and spirit in a place with a bishop and well over a hundred other clergy was, well, hard to describe. Another lay person in the ensemble spoke of “industrial strength” Mass, but it was a very moving experience for all of us.