Three Seasons, Thirty Day Months, and Five Days Left Over

The civil calendar the pharaonic bureaucracy kept was exactly three hundred sixty five days, no more and no less, with no leap day to correct it: three seasons of four months each, Akhet the inundation, Peret the emergence, and Shemu the harvest, every month a fixed thirty days, giving three hundred sixty days closed by five epagomenal days appended at the year's end. Egyptian mythology held these five extra days to be the birthdays of five siblings born to the sky goddess Nut, Osiris, Horus the Elder, Set, Isis and Nephthys in order, a genealogy recorded by Plutarch centuries after the calendar itself was already ancient and reflected in earlier Egyptian sources as well.

Because the true solar year runs about a quarter day longer than three hundred sixty five, the civil calendar drifted against the seasons by a full day every four years and completed an entire cycle back to its starting alignment only once every roughly fourteen hundred sixty years, the interval astronomers call the Sothic cycle after Sopdet, the star Sirius, whose heliacal rising the calendar had originally been set to track before the drift began. A parallel lunar calendar, kept for temple ritual rather than civil administration, ran alongside this fixed count for the whole of its history.