This wonderful tune was composed by Gregory Allegri, about 400 years ago. The piece was composed for the Pope. The Pope was so impressed that he considered the music sacred. He kept it locked in a vault and would listen to it only on Ash Wednesday. Examining the manuscript was strictly forbidden.
Mozart went to Rome to listen to it, and he was able to copy the work from memory. That's why we can listen to it now.
I first heard Miserere at the Anglican Church of St. Mary the Virgin in New York City, where it was performed only on Ash Wednesday and the Wednesday during Holy Week.