Complex sentence: The Festival Te Deum was first performed on 1 May 1872 at the Crystal Palace in a special "Thanksgiving Day" concert organised by the Prince's brother, Alfred, Duke of Edinburgh, who was a friend of Sullivan's and commissioned the piece.
Split into simpler sentences: The Festival Te Deum was first performed on 1 May 1872 at the Crystal Palace in a special "Thanksgiving Day" concert organised by the Prince's brother. The Duke of Edinburgh, who was a friend of Sullivan's, commissioned the piece.