"God called the light Day, and the darkness He called Night. And there was evening and there was morning, a first day."
This inconsistent wording with the next verses where days are referred as: “first, second, etc.” Come to teach us that on that first day, He was absolutely alone. Even the angels were only created on the second day. This was done, so no one may think that Hashem had or needed help to create the world.
Another opinion holds that the only living thing that was created on the first day, was the angel of death. Since it’s a destructive force, no one would think that he had anything to do with the original act of creation.
Others argue and say that Angels were created on the first day.