Prehistory . Current understanding of the earliest development of the Egyptian calendar remains speculative. A tablet from the reign of the
First Dynasty pharaoh Djer (BC) was once thought to indicate that the Egyptians had already established a link between the
heliacal rising of
Sirius ( or
Sopdet, "Triangle"; ,
Sôthis) and the beginning of their year, but more recent analysis has questioned whether the tablet's picture refers to Sirius at all. Similarly, based on the
Palermo Stone,
Alexander Scharff proposed that the
Old Kingdom observed a 320-day year, but his theory has not been widely accepted. Some evidence suggests the early civil calendar had 360 days, although it might merely reflect the unusual status of the five
epagomenal days as days "added on" to the proper year. With its interior
effectively rainless for thousands of years, ancient Egypt was "a gift of the river"
Nile, whose
annual flooding organized the natural year into three broad natural seasons known to the Egyptians as: •
Inundation or Flood (, sometimes anglicized as
Akhet): roughly from September to January. •
Emergence or Winter (''
, sometimes anglicized as Peret''): roughly from January to May. •
Low Water or Harvest or Summer (''
, sometimes anglicized as Shemu''): roughly from May to September.
Parker and others have argued for its development into an observational and then calculated
lunisolar calendar which used a 30 day
intercalary month every two to three years to accommodate the
lunar year's loss of about 11 days a year relative to the
solar year and to maintain the placement of the
heliacal rising of
Sirius within its
twelfth month. No evidence for such a month, however, exists in the present historical record. A second lunar calendar is attested by a
demotic astronomical papyrus dating to sometime after 144 AD which outlines a
lunisolar calendar operating in accordance with the Egyptian civil calendar according to a 25 year cycle. The calendar seems to show its month beginning with the first visibility of the waxing crescent moon, but
Parker displayed an error in the cycle of about a day in 500 years, using it to show the cycle was developed to correspond with the new moon around 357BC. This date places it prior to the
Ptolemaic period and within the native Egyptian
Dynasty XXX. Egypt's
1st Persian occupation, however, seems likely to have been its inspiration. This lunisolar calendar's calculations apparently continued to be used without correction into the
Roman period, even when they no longer precisely matched the observable lunar phases. The days of the lunar month — known to the Egyptians as a "temple month" — were individually named and celebrated as stages in the life of the moon god, variously
Thoth in the
Middle Kingdom or
Khonsu in the
Ptolemaic era: "He ... is conceived ... on
Psḏntyw; he is born on
Ꜣbd; he grows old after
Smdt".
Civil calendar (
bottom) and
Orion (
right). Together, the three brightest stars of the northern winter sky—Sirius,
Betelgeuse (
orange star, upper right), and
Procyon (
upper left)—can also be understood as forming the
Winter Triangle. star chart . The civil calendar was established at some early date in or before the
Old Kingdom, with probable evidence of its use early in the reign of
Shepseskaf (BC,
Dynasty IV) and certain attestation during the reign of
Neferirkare (mid-25th centuryBC,
Dynasty V). It was probably based upon
astronomical observations of
Sirius whose
reappearance in the sky closely corresponded to the average onset of the
Nile flood through the 5th and A recent development is the discovery that the 30-day month of the
Mesopotamian calendar dates as late as the
Jemdet Nasr Period (late 4th-millenniumBC), a time Egyptian culture was borrowing various objects and cultural features from the
Fertile Crescent, leaving open the possibility that the main features of the calendar were borrowed in one direction or the other as well. The civil year comprised exactly 365 days, divided into 12 months of 30 days each and an
intercalary month of five days, which were celebrated as the birthdays of the gods
Osiris,
Horus,
Set,
Isis, and
Nephthys. The regular months were grouped into Egypt's three seasons, which gave them their original names, and divided into three 10-day periods known as
decans or decades. In later sources, these were distinguished as "first", "middle", and "last". It has been suggested that during the
Nineteenth Dynasty and the
Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work. Dates were typically expressed in a
YMD format, with a
pharaoh's
regnal year followed by the month followed by the day of the month. For example, the New Year occurred on The importance of the calendar to
Egyptian religion is reflected in the use of the title "Lord of Years" ('''') for its various creator gods. Time was also considered an integral aspect of
Maat, Following
Censorinus and
Meyer, the standard understanding was that, four years from the calendar's inception, Sirius would have no longer reappeared on the Egyptian New Year but on the next day ; four years later, it would have reappeared on the day after that; and so on through the entire calendar until its rise finally returned to 1460 years after the calendar's inception, an event known as "
apocatastasis". Owing to the event's extreme regularity, Egyptian recordings of the calendrical date of the rise of Sirius have been used by
Egyptologists to fix its calendar and other events dated to it, at least to the level of the four-Egyptian-year periods which share the same date for Sirius's return, known as "tetraëterides" or "quadrennia". but, since the calendar is attested before
Dynasty XVIII and the last date is now known to far predate
early Egyptian civilization, it is typically credited to
Dynasty II around the middle date. Censorinus's date is usually emended to 20July but ancient authorities give a variety of 'fixed' dates for the rise of Sirius. His use of the year 139 seems questionable, as 136 seems to have been the start of the tetraëteris and the later date chosen to flatter the birthday of Censorinus's patron. Perfect observation of Sirius's actual behavior during the cycle—including its minor shift relative to the solar year—would produce a period of 1457 years; observational difficulties produce a further margin of error of about two decades. honoring him and his wife as gods equivalent to the children of
Nut. The reform was resisted by the Egyptian priests and people and was abandoned.
Coptic calendar Egyptian scholars were involved with the establishment of
Julius Caesar's
reform of the
Roman calendar, although the Roman priests initially misapplied its formula and—by counting inclusively—added leap days every three years instead of every four. The mistake was corrected by
Augustus through omitting leap years for a number of cycles until AD4. As the personal ruler of
Egypt, he also imposed a reform of its calendar in 26 or 25BC, possibly to correspond with the beginning of a new
Callipic cycle, with the first leap day occurring on 6 Epag. in the year 22BC. This "Alexandrian calendar" corresponds almost exactly to the
Julian, causing 1
Thoth to remain at 29August except during the year before a Julian leap year, when it occurs on 30August instead. The calendars then resume their correspondence after 4Phamenoth/ 29February of the next year. ==Months==