Is it okay to play the flute in a SSPX Church on Christmas eve before the Midnight Mass? My daughter has been asked to play but I am not too sure if I should allow it.
Pbax, if you wish to be a Catholic parent rather than a Puritan one, ignore the advice from commenters whose understanding of the rubrics of musical performance in church, both outside of and during Mass, leaves much to be desired. The most charitable way to put the matter is that one who says that the use in church of woodwind instruments—or indeed any instruments other than the organ—is simply prohibited is confusing economic necessity with orthodoxy of practice.
On the other hand, if you think that breaking your daughter's heart at Christmas just for the heck of it is the way to show one and all that you are a virtuous parent, then by all means deny her permission to play. For good measure, why not break her flute in half and throw it into the garbage?
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Look at the situation another way. Are you prepared to entertain the notion that the masses, vespers, and liturgical motets composed by Dufay, Ockeghem, Josquin, Victoria, Palestrina, Allegri, Monteverdi, and others continuing down through the nineteenth century, many (not all) of which include instrumental parts for ensembles of five to fifty instruments, violated canonical norms and should be regarded as proto-Modernist in character? If you are, you are wedding your mind and spirit to misinformation and ignorance.
It is undoubtedly true that for the past six hundred years the use of instruments in addition to the organ has been associated with diocesan cathedrals, basilicas, and other grand church and chapel structures.
* Yet the absence of any instrument save the organ from smaller, more typical parish and community churches has always had far more to do with (1) the lack of space for anything else, (2) the often prohibitive considerations of cost and time associated with musical preparation, or (3) both. Yet even in the poorest churches during the poorest epochs, the celebration of Christmas has always involved the maximizing of festivity. Thus, the use of a flute on Christmas Eve,
especially in a context apart from Mass, should not become an occasion for inappropriate, self-induced scrupulosity. Rather, it should be applauded as activity whereby Catholics of today unite themselves with the activity of Catholics throughout the Church's history.
It is also true that the specifics of musical praxis have varied from diocese to diocese and prelate to prelate. Political considerations of a temporal nature have also come into play—in fact, for most of history such considerations have frequently been a determining factor for many church-related matters, music being just one of them. But what these variations of custom and practice ought to tell you is that people who point to stern and universal pre-Vatican II regulations are trying to make you their companions in misinformation. Even the regulations of Pope Saint Pius X, which were a response to perceived abuses in a time (like the present) where both the faith and the liturgy were widely abused and adulterated, left the door wide open to exceptions of all sorts, as actual events and practices of his own time confirm. At Saint Peter's Basilica, very grand masses and solemn vespers were the rule rather than the exception back then, and so they have remained.
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Most of what I have learned during the past fifty-plus years about the nexus of music and the liturgy was acquired the old-fashioned way: through the reading of books and through the instruction of specialists in this field. Those are still the best ways to go about learning a thing or two on this topic, a fortiori now that so few priests and religious (including Trads) know anything at all about it. (In fairness, most of them are too busy with the fundamentals of faith to keep up on their historical liturgical studies.) But focused Internet research can also yield surprisingly useful information in a matter of hours. The trick is not to be fooled into following the lead of people whose own lack of knowledge rivals your own!
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*Europe formerly had several hundred private chapels belonging to the nobility and, later, the just plain rich; a great many of these people, both clerics and laymen, employed composers who were kept especially busy during Advent, Christmastide, and Holy Week. Many abbey and convent chapels—excluding those, like the Carthusians, whose focus was severely contemplative, of course—also made regular use of instruments and of instrumental accompaniments to choral singing,
especially at times of heightened rejoicing, the most notable of which are Christmas and Easter.