The ancient civil Egyptian calendar had a year that was 365 days long and was divided into 12 months of 30 days each, plus five extra days (epagomenae, from Greek ἐπαγόμεναι) at the end of the year. The months were divided into three weeks of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events therefore "wandered" through the calendar, it has been referred to as the annus vagus, or "wandering year".
Early use 
A tablet from the reign of First Dynasty King Djer (c. 3000 BC) was conjectured by early Egyptologists to indicate that the Egyptians had already established a link between the heliacal rising of Sirius (Egyptian Sopdet, Greek Σείριος Seirios) and the beginning of the year. However, more recent analysis of the pictorial scene on this tablet has questioned whether it actually refers to Sothis at all. Current knowledge of this period remains a matter more of speculation than of established fact.
The Egyptians may have used a luni-solar calendar at an earlier date, with the intercalation of an extra month regulated either by the heliacal rising of Sothis or by the inundation of the fields by the Nile. The first inundation according to the calendar was observed in Egypt's first capital, Memphis, at the same time as the heliacal rising of Sirius. The Egyptian year was divided into the three seasons of akhet (Inundation), peret (Growth - Winter) and shemu (Harvest - Summer).
The heliacal rising of Sothis returned to the same point in the calendar every 1,460 years (a period called the Sothic cycle). The difference between a seasonal year and a civil year was therefore 365 days in 1,460 years, or one day in four years. Similarly, the Egyptians were aware that 309 lunations nearly equaled 9,125 days, or 25 Egyptian years, which was later used in the construction of a secondary lunar calendar that did not depend on observations.
For much of Egyptian history, the months were not referred to by individual names, but were rather numbered within the three seasons. As early as the Middle Kingdom, however, each month had its own name. These finally evolved into the New Kingdom months, which in turn gave rise to the Hellenized names that were used for chronology by Ptolemy in his Almagest, and by others.
Copernicus constructed his tables for the motion of the planets based on the Egyptian year because of its mathematical regularity. The convention amongst modern Egyptologists is to number the months consecutively using Roman numerals.
Ptolemaic and Roman 
According to Roman writer Censorinus, the Egyptian New Year's Day fell on July 20 in 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 BC, and the one before that was 2782 BCE. This latter date has been postulated as the time when the calendar was invented, but Djer's reign preceded that date.
In 238 BCE, the Ptolemaic rulers decreed that every 4th year should be 366 days long rather than 365. The Egyptians, most of whom were farmers, did not accept the reform, as it was the agricultural seasons that made up their year. The reform eventually went into effect with the introduction of the "Alexandrian calendar" by Augustus in 26/25 BCE, which included a 6th epagomenal day for the first time in 22 BCE. This almost stopped the movement of the first day of the year, 1 Thoth, relative to the seasons, leaving it on 29 August in the Julian calendar except in the year before a Julian leap year, when a 6th epagomenal day occurred on 29 August, shifting 1 Thoth to 30 August.
Reformed calendar 
The reformed Egyptian calendar continues to be used in Egypt as the Coptic calendar of the Egyptian Church and by the Egyptian populace at large, particularly the fellahin, to calculate the agricultural seasons. Contemporary Egyptian farmers, like their ancient predecessors, divide the year into three seasons, namely winter, summer and inundation. It is also associated with local festivals such as the annual Flooding of the Nile and the ancient Spring festival sham en nisim.
The Ethiopian calendar is based on this reformed calendar but uses Amharic names for its months and uses a different era. The French Republican Calendar was similar, but began its year at the autumnal equinox. British orrery maker John Gleave represented the Egyptian calendar in a reconstruction of the Antikythera mechanism.
|No.||Seasonal Names||Middle Kingdom||New Kingdom||Greek||Coptic||Egyptian Arabic|
|Latin script||Greek script||Latin script||Arabic script|
|I||First of Akhet||Tekh||Dhwt||Thoth||Θώθ||Thout||Tout||توت|
|II||Second of Akhet||Menhet||Pa-n-ip.t||Phaophi||Φαωφί/Φαῶφι||Paopi||Baba||بابه|
|III||Third of Akhet||Ḥwt-ḥwr||Ḥwt-ḥwr||Athyr||Ἀθύρ||Hathor||Hatour||هاتور|
|IV||Fourth of Akhet||Ka-ḥr-ka||Ka-ḥr-ka||Choiak||Χοιάκ/Χοίακ||Koiak||Kiahk||(كياك (كيهك|
|V||First of Peret||Sf-bdt||Ta-'b||Tybi||Τυβί/Τῦβι||Tobi||Touba||طوبه|
|VI||Second of Peret||Rekh wer||Mḫyr||Mechir||Μεχίρ/Μεχείρ||Meshir||Amshir||أمشير|
|VII||Third of Peret||Rekh neds||Pa-n-amn-htp.w||Phamenoth||Φαμενώθ||Paremhat||Baramhat||برمهات|
|VIII||Fourth of Peret||Renwet||Pa-n-rnn.t||Pharmouthi||Φαρμουθί/Φαρμοῦθι||Paremoude||Baramouda||برموده|
|IX||First of Shemu||Hnsw||Pa-n-ḫns.w||Pachon||Παχών||Pashons||Bashans||بشنس|
|X||Second of Shemu||Hnt-htj||Pa-n-in.t||Payni||Παϋνί/Παῦνι||Paoni||Ba'ouna||بئونه|
|XI||Third of Shemu||Ipt-hmt||Ipip||Epiphi||Ἐπιφί/Ἐπείφ||Epip||Abib||أبيب|
|XII||Fourth of Shemu||Wep-renpet||Msw-r'||Mesore||Μεσορή||Mesori||Mesra||مسرا|
See also 
- Winlock, "Origin of the Ancient Egyptian Calendar," Proceedings of the American Philosophical Society, 83 (1940): 447-64.
- Marshall Clagett. Ancient Egyptian Science: A Source Book (1989) 10–11.
- Parker, Calendars of Ancient Egypt, pp.30-2.
- Parker, Calendars of Ancient Egypt, pp.13-29.
- Alexandrian reform of the Egyptian calendar
- Where two spellings are offered, the first uses the "Greek" spelling and accentuation (taken from F. Montanari, Vocabolario della Lingua greca 1995), and the second the reconstructed Egyptian accentuation (from P.W. Pestman, The new papyrological primer, 1990).
- Grimal, Nicolas. A History of Ancient Egypt. Oxford: Blackwell Publishers, 1997.
- Parker, Richard A. The Calendars of Ancient Egypt, Studies in Ancient Oriental Civilization, 26. Chicago: University of Chicago Press, 1950.
- Shaw, Ian. ed. The Oxford Illustrated History of Ancient Egypt. Oxford: Oxford University Press, 2000.
- Watterson, Barbara. The Egyptians. Oxford: Blackwell Publishers. 1997.
- Youssef, Ahmad Abdel-Hamid. From Pharaoh's Lips: Ancient Egyptian Language in the Arabic of Today. Cairo: American University in Cairo Press, 2003.
- Date Converter for Ancient Egypt
- Calendrica Includes the Egyptian civil calendar with years in Ptolemy's Nabonassar Era (year 1 = 747 BC) as well as the Coptic, Ethiopic, and French calendars.
- CIVIL4.0 is a tiny DOS program (Zipped, 25kB) to convert Egyptian Civil dates into Julian or Gregorian dates, BC and AD.
- Detailed information about the Egyptian calendars, including lunar cycles