Happy Feast of the Immaculate Conception!
When I was in seminary in Rome, the rector called everyone to the chapel and personally taught all the men this chant.
Here’s another version which I love, by Maurice Durufle. It took a while to find one that didn’t rush it.
I think that if some choir were to surprise me with this – sung well – on a Marian feast during Mass, I might either burst into tears or expire on the spot.