Miserere mei, Deus ("Have mercy on me, O God") are the opening words of Psalm 51 and have led to it being traditionally referred to as the Miserere, which is the name it is often known by in musical settings. This 17th century version by composer Gregorio Allegri is one of the best-known settings of the Miserere. Listen to the solo when the Soprano hits the C above the staff. It is one of the most hauntingly moving moments in choral music.
Today begins the Triduum (the three days from the evening of Holy Thursday to the evening of Easter Sunday) of Holy Week. It is a time of prayer and deep devotion, and there is no better musical piece than the Miserere to preface the three days that changed the world.