The Egyptians were probably the first to adopt a mainly solar calendar. They noted that the Dog Star, Sirius, reappeared in the eastern sky just before sunrise after several months of invisibility. They also observed that the annual flooding of the Nile River came soon after Sirius reappeared.
They used this combination of events to fix their calendar and came to recognize a year of 365 days, made up of 12 months each 30 days long, and an extra five days added at the end. But they did not allow for the extra fourth of a day, and their calendar drifted into error. According to the famed Egyptologist J. H. Breasted, the earliest date known in the Egyptian calendar corresponds to 4236 B.C. in terms of the Gregorian calendar.
The ancient Egyptians originally employed a calendar based upon the Moon, and, like many peoples throughout the world, they regulated their lunar calendar by means of the guidance of a sidereal calendar. They used the seasonal appearance of the star Sirius (Sothis); this corresponded closely to the true solar year, being only 12 minutes shorter. Certain difficulties arose, however, because of the inherent incompatibility of lunar and solar years.
To solve this problem the Egyptians invented a schematized civil year of 365 days divided into three seasons, each of which consisted of four months of 30 days each. To complete the year, five intercalary days were added at its end, so that the 12 months were equal to 360 days plus five extra days.
This civil calendar was derived from the lunar calendar (using months) and the agricultural, or Nile, fluctuations (using seasons); it was, however, no longer directly connected to either and thus was not controlled by them. The civil calendar served government and administration, while the lunar calendar continued to regulate religious affairs and everyday life.
In time, the discrepancy between the civil calendar and the older lunar structure became obvious. Because the lunar calendar was controlled by the rising of Sirius, its months would correspond to the same season each year, while the civil calendar would move through the seasons because the civil year was about one-fourth day shorter than the solar year.
Hence, every four years it would fall behind the solar year by one day, and after 1,460 years it would again agree with the lunisolar calendar. Such a period of time is called a Sothic cycle.
Because of the discrepancy between these two calendars, the Egyptians established a second lunar calendar based upon the civil year and not, as the older one had been, upon the sighting of Sirius.
It was schematic and artificial, and its purpose was to determine religious celebrations and duties. In order to keep it in general agreement with the civil year, a month was intercalated every time the first day of the lunar year came before the first day of the civil year; later, a 25-year cycle of intercalation was introduced.
The original lunar calendar, however, was not abandoned but was retained primarily for agriculture because of its agreement with the seasons. Thus, the ancient Egyptians operated with three calendars, each for a different purpose.
The only unit of time that was larger than a year was the reign of a king. The usual custom of dating by reign was: "year 1, 2, 3 . . . , etc., of King So-and-So," and with each new king the counting reverted back to year One. King lists recorded consecutive rulers and the total years of their respective reigns.
The civil year was divided into three seasons, commonly translated: Inundation, when the Nile overflowed the agricultural land; Going Forth, the time of planting when the Nile returned to its bed; and Deficiency, the time of low water and harvest.
The months of the civil calendar were numbered according to their respective seasons and were not listed by any particular name--e.g., third month of Inundation--but for religious purposes the months had names. How early these names were employed in the later lunar calendar is obscure.
The days in the civil calendar were also indicated by number and listed according to their respective months. Thus a full civil date would be: "Regnal year 1, fourth month of Inundation, day 5, under the majesty of King So-and-So." In the lunar calendar, however, each day had a specific name, and from some of these names it can be seen that the four quarters or chief phases of the Moon were recognized, although the Egyptians did not use these quarters to divide the month into smaller segments, such as weeks.
Unlike most people who used a lunar calendar, the Egyptians began their day with sunrise instead of sunset because they began their month, and consequently their day, by the disappearance of the old Moon just before dawn.
As was customary in early civilizations, the hours were unequal, daylight being divided into 12 parts, and the night likewise; the duration of these parts varied with the seasons. Both water clocks and sundials were constructed with notations to indicate the hours for the different months and seasons of the year. The standard hour of constant length was never employed in ancient Egypt.
Reference: Encyclopedia Britannica
The ancient civil Egyptian Calendar, known as the Annus Vagus or Wandering Year, had a year that was 365 days long, consisting of 12 months of 30 days each, plus 5 extra days at the end of the year. The months were divided into 3 "weeks" of ten days each.
This calendar was in use by 2400 BCE, and possibly before that. It was used throughout antiquity. It was used by astronomers in the Middle Ages because of its mathematical regularity.
The Egyptian calendar was simple, but it is neither a lunar nor a solar calendar. Months do not correspond to lunar months, and years do not correspond to solar years. The Egyptians were aware of this, and calculated their seasonal year by the stars, to be the time between successive heliacal risings of the star Sirius (which the Egyptians called Sothis).
The heliacal rising of Sothis returned to the same point in the calendar every 1460 years (a period called the Sothic cycle). The difference between a seasonal year and a civil year was therefore 365 days in 1460 years, or 1 day in 4 years. Similarly, the Egyptians were aware that 309 lunations nearly equalled 9125 days, or 25 Egyptian years, which was likely used in the construction of a secondary lunar calendar.
According to the Roman writer Censorinus, the Egyptian New Year's Day fell on July 20 on the Julian Calendar in 139 CE, which was a heliacal rising of Sirius in Egypt. From this it is possible to calculate that the previous occasion on which this occurred was 1322 BCE, and the one before that was 2782 BCE. This latter date has been postulated as the time when the calendar was invented, though earlier historians tended to push it back another whole cycle, to 4242 BCE.
In 238 BCE, the Ptolemaic rulers decreed that every 4th year should be 366 days long rather than 365.
That practice was not followed, however, until the introduction of the "Alexandrian Calendar" in 22 BCE by Augustus.
British orrery maker John Gleave represented the Egyptian calendar in a reconstruction of the Antikythera Mechanism, an ancient mechanical analog computer (as opposed to digital computer) designed to calculate astronomical positions. It was discovered in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to about 80 BC.
For most of Egyptian history, the months were not given individual names but rather were numbered within the three seasons of Akhet (Inundation), Proyet (Emergence), and Shomu (Harvest).
During the New Kingdom, however, each month was given its own name. These eventually evolved into the Hellenistic names that are still used today by the Coptic Church. The convention amongst modern Egyptologists is to number the months consecutively using Roman numerals.