Psalm 50 is one of the Seven Penitential Psalms. The others are Psalms 6, 31, 37, 101, 129 and 142. The lenten devotion of reciting the Seven Penitential Psalms was common throughut Christendom and deserves to be common again.
The setting in this video is that of Allegri and is the most famous musical setting of Psalm 50. It is one of the three settings (that of Abbot Giuseppe Baini on Wednesday, that of Tommaso Bai on Thursday, and Allegri's on Friday) that were annually sung during the Tenebrae in the Papal Chapel.
These settings acquired a considerable reputation for mystery and inaccessibility because the Holy See forbade the making of copies of the music held by the Sistine Chapel Choir, threatening any publication or attempted copy with excommunication. However, a young Mozart attended the ceremonies of Holy Week at the Vatican in 1770 and transcribed them from memory afterwards. Although, it must be admitted that the version that emerged is a conflation of the settings of Allegri and Bai.