I've only just learned it.
At Eastern Rite liturgies I've always thrilled a bit at the phrase, "I will not reveal Your Mysteries to your enemies," as if we were suddenly plunged by the universality and sacred timeliness/timelessness of the Sacrifice into penal times, or the first century and were willing to die rather than betray the goings-on in the catacombs to the authorities.
One doesn't see it much in the West, although the sheet music from which I first learned the Bruckner Locus Iste translated sacramentum that way, IIRC.
This is, I promise you, not about aesthetic snobbery, but the music that will happen tonight at any of the Roman Catholic churches within reach will be so jarringly bad or inappropriate that I was casting about for an alternative, and my default Byzantine parish seems not to be having any liturgy tonight, can that be possible?
(Here, I can prove, at least to my own satisfaction that it's not snootiness on my part - if it were not a thousand miles away I would attend an Extraordinary Form Mass I used to hear with some regularity, where the propers that are not sung recto tono or to a psalm tone would be unrecognizable they would be rendered so badly, by a choir that included on singer with a voice like an electric cheese grater. The appropriate done badly by worshipers giving their best is more fitting than the inappropriate done well by worshipers giving their favorites.)
Anyway, I found a church, (of a rite that I don't know well at all,) whose bulletin gives a time for Liturgy of Thursday of Mysteries.
(And between Holy Orders and Eucharist, and washing of the feet - do Easterners do that? dont know - our evening will be filled with Mystery.)
Was this Father Hardon?
"A mystery is not a truth about which we can know nothing. It is a truth about which we cannot know everything."