The fifth listed Mass For Various Needs And Occasions is “For a Council or a Synod.”
Every now and the Church convenes a synod. It may be a collaboration of bishops, as described and legislated here. It may occur within a diocese, as described here. These are significant and important meetings, on whichever level they are held–certainly worthy of significant prayer among the participants and outside of ther meeting halls.
The Entrance Antiphon is suggested by Colossians 3:16:
And over all things put on love,
which is the bond of perfection,
and let the peace of Christ rule in your hearts.
No Psalm is given to accompany this text. This is part of a larger pericope that encompasses verses 12 through 17. If not that passage, it would seem the “canticle” of 1 Corinthians 13 is a possibility. If pressed for a Psalm text, I would suggest the 100th.
The Communion Antiphon is Ubi Caritas, and it would seen that entire ancient hymn is fitting for the celebration of a synod.
The two options for the Opening Prayer speak of good governance. The presider at Mass may choose to pray for “a pour(ing) out … a spirit of truth, understanding, and peace … know(ledge of) wha is pleasing to (God)” and “then pursue it with all … strength.” The alternate is a petition to “endow with a spirit of wisdom those (with) authority to govern, that … people may be led to know the truth more fully and to grow in holiness …”
Anyone reading ever participate in a synod? If so, did you celebrate the Mass for the occasion?
In the GIRM, sections 368-378 cover the universal regulations on their use. You can check our brief comments here and here and here. The USCCB’s unannotated text on the matter is here.