Re: Advent Penitential Service - what is it?
Well, we have no way of knowing exactly what will happen at these services but generally what happens is that there will be both communal and individual parts to the service.
In many, if not most, locations the priests from neighboring parishes, schools, seminaries, etc, all show up. Usually everyone will gather for prayer. There will probably be scripture readings. There will probably be some kind of examination of conscience. There will likely be some sort of communal act of contrition.
Once the communal part of the service has concluded, the priests will all be excused to go to the confessionals, reconciliation rooms, quiet corners of the church, classrooms, etc. in order to hear private confessions. In many cases this will mean face to face confession since there are frequently more priests than there are private places.
In my parish is is customary for the accompanist to play quiet music, partly to set the mood, but mostly to mask the voices of those confessing since we usually have several priests in the main body of the church building.
When all (or most) of the individual confessions are finished, some parishes will have everyone gather again for some sort of closing prayer. In other cases people are free to leave once they have confessed and possibly stayed for private prayer.