The Greek goddess Hemera was one of the Protogenoi, i.e. a primordial deity. She represented the personification of the day/daylight. She was one of the daughters of Erebus and Nyx and, as such, the sister of Aether (the Atmosphere), whom she later married.
Her Roman name was Dies (Day). Pseudo-Hyginus says she was a daughter of Chaos and a sister of Nox (Night), not her daughter.
Nyx would bring night on earth, by drawing a veil of darkness between the light of the upper atmosphere and the normal air that people on Earth breathed. In the morning, Hemera would come and draw it back, dispersing the darkness and allowing light to reach the earth. (In the early cosmogony, light and darkness were divine substances and they were not caused by the presence or absence of the sun).
Both goddess Hemera and Nyx lived in Tartarus, but they only briefly met twice a day, when one of them was leaving, while the other one was coming back home. So, while one of them was travelling above the earth, the other one was at home, waiting for her turn.
Later she was identified with Eos, and Pausanias describes images of Eos, but he mistakenly identifies her as Hemera. Philostratus mentions the statue of Memnon, in Aethiopia, which would greet Hemera, his mother, in the morning, and would mourn her departure in the evening. In fact, Memnon was the son of Eos, so Philostratus thought the two goddesses were one and the same.
Philostratus describes a painting representing the fall of Phaeton, Helios' son. He wanted to drive his father's chariot, but he couldn't maintain a steady grip and he fell. The whole world plunged into darkness, because Nyx came and drove goddess Hemera (the light) away.