“Let all mortal flesh keep silence” is a hymn about Holy Communion. But now it keeps showing up as a Christmas carol! That’s how it’s presented in Christmas concerts, holiday recordings, and on many of the renditions on Youtube. But it is “an ancient chant of Eucharistic devotion.”
I can see how it would shift over to the Christmas canon. It has a beautiful, otherworldly melody of the same sort associated with Christmas music. It talks about how “Christ our God to earth descendeth” and that He was “born of Mary.” But the point is that “He will give to all the faithful/His own self for heavenly food,” “in the body and the blood.”
But this is quite fitting to associate Holy Communion with Christmas and vice versa. What other Communion hymns could work as Christmas hymns? What other Christmas songs could work as Communion songs?