Hemera is the personified Day in Greek cosmogony. She appears most clearly in genealogical poetry rather than in a large body of independent myths or a major public cult.
Birth of Day
In Hesiod’s Theogony, Night gives birth to Aether and Day after union with Erebus. Hemera therefore belongs to the earliest cosmic generations, before the Olympian order established under Zeus.
Hesiod describes Night and Day passing one another at the threshold of a distant house. One enters as the other leaves, so they never occupy the world at the same time. The image explains the daily alternation of darkness and light through divine movement.
Hemera in later sources
Other ancient genealogies arrange the first cosmic beings differently. Some Orphic materials give Day a different place or parentage. These variations reflect separate poetic systems rather than an error that must be forced into one family tree.
Modern lists sometimes describe Hemera as a major goddess of daylight with an extensive personality, symbols, and worship. The surviving evidence is much narrower. She is best understood as a cosmic personification whose importance comes from the structure of early Greek creation poetry.