The singer Billie Holiday was also known as "Lady Day".

The answer is true because Billie Holiday, a renowned jazz and blues singer, earned the nickname "Lady Day" during her career. The term "Lady" was commonly used to address and honor female singers in the jazz community, and "Day" was taken from her given name, Billie Holiday. The nickname became widely known and used by fans, peers, and the media as a homage to her incredible talent and unique vocal style, making it a significant part of her musical identity.