"Lonely Day" is the second single from the 2005 album Hypnotize by Armenian American metal band System of a Down. It was written by guitarist Daron Malakian, who also provides lead vocals on the track. The song received a nomination for Best Hard Rock Performance at the 49th Annual Grammy Awards.

Answer this question: who sings lonely day system of a down? Let's give stream of consciousness first:
The important part for answering this question is: It was written by guitarist Daron Malakian, who also provides lead vocals on the track.
So, the final answer is guitarist Daron Malakian.