The Ancient Egyptian calendar, which dates back to before 3000 BC, is the basis of the calendar that is used in Western countries today. It is a solar-based calendar dividing the year into 365 or 366 days and 3 seasons of four months each (the one we have now has four seasons divided into three months each), according to the Egyptian climatic changes.
The first season is the inundation season (Akhet) consisting of the months of Djehwty (Tut), Ba en Ibat (Babe), Hwt-hr (Hathor) and Ka-her-ka (Kiak).
The second season is cultivation season (Peret) and the months are Ta-ibt (Tuba), Pa-en-mechr (Amshir), Pa-en imnhotbu (Peremhat) and Pa-en-renenuter (Paramuda). The third is summer or harvesting (Shemu) seasonwith the months of Pa-en-khons (Pashons), Pa-en-Inet (Peunet), Ebib (Abib) and Mesut-re (Mesre).
The Ancient Egyptian months were all 30 days long, making a total of 360 days. The Egyptians would then complete the year by adding five days celebrating of their deities. The first day was for Osiris, the second for Horus, the third for Seth, the forth for Isis and the last day for Nephthys.
The Ancient Egyptians noticed that, shortly after the brightest star, Sirius appears in the sky (named Sothis at the time of the Pharaohs), the flood would follow, now known as the heliacal rising of the Sirius. This happens around 19 July. Based on that, as we can see, the length of the year was decided astronomically and not by calculations.
In the Egyptian calendar the week was defined in a different way. The seven days week, that we use today, were based on religious considerations, not astronomical ones, like the movements of the sun or the moon. The notion of the week was related either to the Holy Bible (the Old Testament says that the world was created in seven days) or to the Babylonian myth that identified seven heavens – the sun, the moon and the five planets known at the time; Mercury, Venus, Mars, Jupiter and Saturn.
This is reflected in the naming of the days of the week, especially in French; Lundi (in French ‘lune’ means moon), Mardi (Mars), Mercredi (Mercury), Jeudi (Jupiter) and Vendredi (Venus). In English Saturday is named after Saturn, Sunday after the Sun and Monday after the Moon.
The Ancient Egyptians had a more mathematical and organized way of thinking. They divided the month that is 30 days into three equal parts of ten days each, which we nowadays call the Decans. They therefore divided the year into 36 Decans. The Ancient Egyptians also divided the day and the night each into 12 equal hours irrespective of the length of the day or the night.
This calendar was in use by the Ancient Egyptians and is still used by Egyptian Copts to this day. The Egyptian calendar was adopted later on by the Roman Empire (including the use of Decans), yet they modified the length of the months by distributing the five additional days among five of the months of the year.
The Julian calendar was based on this, then several modifications were added to make what we have now, known as the Gregorian calendar. So the international calendar we use today is basically the calendar developed by the Ancient Egyptians more than 5,000 years ago, with a few changes.
