Traditionis Custodes is being “applied” in ways that go well beyond what would be required by the letter of the law (such as it is; Fr. Réginald-Marie Rivoire in his masterful canonical tract has shown that it is bad law and worse theology; see also my article on newly-ordained priests and permission to offer the usus antiquior). One such way is when bishops attempt to redefine “private Mass” as a Mass at which only a priest and a server are present, and no one else.
Let’s begin with a preliminary canonical matter. If a bishop merely tells his priests that this will be his policy, or has it communicated to them in an informal way, then it is neither valid nor legally enforceable, the reason being given in a series of canons:
Can. 49. A singular precept is a decree which directly and legitimately enjoins a specific person or persons to do or omit something, especially in order to urge the observance of law.
Can. 51. A decree is to be issued in writing, with the reasons at least summarily expressed if it is a decision.
Can. 54. §1. A singular decree whose application is entrusted to an executor takes effect from the moment of execution; otherwise, from the moment it is made known to the person by the authority of the one who issued it. §2. To be enforced, a singular decree must be made known by a legitimate document according to the norm of law.
Can. 55. Without prejudice to the prescripts of cann. 37 and 51, when a very grave reason prevents the handing over of the written text of a decree, the decree is considered to have been made known if it is read to the person to whom it is destined in the presence of a notary or two witnesses. After a written record of what has occurred has been prepared, all those present must sign it.
What is to be gathered from these canons is that the bishop would have had to present such a limitation on the rights of a priest in writing and properly promulgate it. If a bishop intends to ban something that a priest is otherwise entitled to, he must issue it in writing, because it has to be the sort of thing capable of being challenged by those affected by it. Otherwise, it would just be a form of bullying: “You gotta do this because I say so,” with no paper trail. Now, in the case at hand (where a bishop attempts to redefine a private Mass), what right of a priest would be being infringed?
Can. 906. Except for a just and reasonable cause, a priest is not to celebrate the Eucharistic sacrifice without the participation of at least some member of the faithful.
Note that Can. 906 normally requires that there be “at least some member of the faithful,” which is deliberately open-ended: it could logically and legally include several people, indeed it could include a large church packed to the rafters. This remains true for a Mass that an unimpeded priest offers on any day of the week in any legitimate place for any legitimate reason. That would include a Mass held, for appropriate reason, in a side chapel, at a school or a retreat center, in a rectory chapel, at a house, etc.
READ ON BELOW…New Liturgical Movement: Can a Bishop Restrict a “Private Mass” in the Usus Antiquior to a Priest and a Server?