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The Demon Days are the last five days in a year and are considered bad luck by Ancient Egyptians. People did not take risks or do anything dangerous or important on these days. To protect themselves they would often wear charms. In addition, it is impossible to create portals on these days, forcing magicians to travel by other means.
The Demon Days were created during the Age of the Gods, when a year was originally three-hundred and sixty days long. However, when Ra heard a prophecy saying that a child of Nut and Geb would overthrow him, he forbade the two to have any children on the three-hundred and sixty days of the year. Desperate, Nut (Thoth in the original myths) gambled with the moon god, Khonsu, and won enough moonlight from him to create five more days. She gave birth to one child each day, resulting in Osiris on December 27, Horus on the 28th, Set on the 29th, Isis on the 30th, and lastly Nephthys on the 31st. In punishment, Ra forced Geb and Nut apart for all eternity, forcing Nut to the sky and Geb down below on the earth. Therefore, Geb is the earth god and Nut the sky goddess. Their own father Shu, the wind god, enforces this punishment by blowing them apart.
Although the Demon Days are considered bad luck by mortals, the gods born on these days are especially powerful on their birthdays. The god Set tried to destroy North America at sunrise on his birthday, the height of his powers.
The Demon Days, though being the 27th through the 31st, actually start at sunset on the 26th. During this time, portals can't be opened by magicians starting at sunset on the 26th, but gods of magic like Isis and Thoth can along with godlings who are fully merged with their gods such as Sadie Kane after merging with Isis or Carter Kane with Horus.