"Lovely Day" is a song by American soul and R&B singer Bill Withers. Written by Withers and Skip Scarborough, it was released on December 21, 1977 and appears on Withers' 1978 album Menagerie.

Based on this passage, who sings the song it's going to be a lovely day? Let's give stream of consciousness first:
To answer the question, consider the following: "Lovely Day" is a song by American soul and R&B singer Bill Withers.
So, the final answer is Bill Withers.