The seminarians start chanting around 0:53 in this YouTube video. I'd like to learn this chant but I do not know what it's called, so if anyone can help me it would be great!
It is a very beautiful Introit chant. You might really enjoy the hymn version of "Rorate Caeli". It's in many hymnals, but the Nova Organi Harmonia (free online) is through-composed, every verse a different accompaniment. We do it every Advent IV during Communion.
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