A calendar that endured for five millennia, carrying the mathematical regularity of astronomy, structured by decans and epagomenal days.
The Egyptian civil calendar is a system designed five millennia ago, and it forms the backbone of modern astronomy. Almost every ancient writer who serves as a source today used this calendar. Anyone reading classical sources will inevitably encounter it. To locate a birth record or to convert an observation date into modern time, a conversion table is essential.
Of course, one reason I chose to reconstruct this calendar is that it was also considered sacred. The year began when Sopdet (Sirius), after seventy days of invisibility, reappeared on the eastern horizon just before dawn. This heliacal rising heralded the flood of the Nile. The three seasons—Akhet (inundation), Peret (emergence), Shemu (harvest)—were born of this cycle. Thirty-six decans divided the night into twelve hours. The five epagomenal days added at the year's end were the birthdays of Nut's five children (Osiris, Horus, Set, Isis, Nephthys), regarded as a sacred threshold standing outside the protective structure of the calendar.
This tool maps the Egyptian civil calendar onto Gregorian dates for the year you choose. It calculates your position in the Sothis cycle, shows where Wepet Renpet falls, lists the seasonal correspondences of the twelve months, and opens the map of the thirty-six decans. You can see which ten-day stretch each decan governs, along with its Egyptian star group and the zodiac-degree correspondence it received in the Hellenistic period. While using this calendar, remember that Egyptian mythology is not merely a museum memory but a tradition that is still ritually practiced and alive.
Heliacal rising of Sopdet. The year begins, the Nile flood approaches.
The civil year is 365 days; the actual sidereal year is about 365.25. This quarter-day difference creates a one-day drift every four years. After 1460 Julian years, the calendar returns to its starting point. The Greeks called this apokatastasis (rebirth). Censorinus, in AD 139, recorded that the heliacal rising of Sopdet coincided with Thoth 1.
The year consists of twelve months, each of thirty days, each of three decans. Thirty-six decans × ten days = three hundred sixty; five epagomenal days are added on top. The oldest known visual record of this system is the Senmut ceiling, dated to circa 1473 BCE. Hellenistic astrology inherited this structure, binding each decan to a ten-degree slice of the zodiac. The decan system we still use today comes from here.
At the year's end, outside the months, there are 5 intercalary days. These are the birthdays of Nut's five children: Osiris, Horus, Set, Isis, and Nephthys. The third day (Set's birth) was considered the most inauspicious. The fifth day (Nephthys) was the eve of the new year.
Cairo Papyrus 86637 (~1244-1163 BCE) is a daily-life calendar containing favorable/unfavorable judgments, mythological references, and behavioral recommendations for each day. It is the most complete surviving example of a hemerology. The Cairo Papyrus was used in the reconstruction of this calendar.