Egyptian calendar

Keeping track of hours, days, and months was of the utmost importance to the Egyptians, as there were many sacred occasions that had to be observed in order to maintain the spiritual bond with the gods and with the dead. The task was complex and crucial enough to require the complete attention of specially trained priests whose duty it was to mark the passage of time.

The new year started with the inundation of the Nile, presaged by the first appearance of the star Sothis (Sirius). The Egyptian calendar followed the lunar model, with 12 months of 30 days each divided into three seasons. The difference between the solar and lunar year was compensated for by the presence of the five Epagomenal Days, the sacred days commemorating the births of Asar (Osiris), Heru the Elder (Horus), Suti (Set), Ast (Isis), and Nebt-Het (Nepthys).

The calendar is divided into the seasons and months given below. I've also mapped the Egyptian months to the Gregorian calendar, for the curious.

Akhet (Season of the Inundation)
Djewhty (June 21 to July 20)
Paopi (July 21 to Aug 19)
Athyr (Aug 20 to Sept 18)
Khoiak (Sept 19 to Oct 20)

Proyet (Season of Planting)
Tybi (Oct 21 to Nov 19)
Mekhir (Nov 20 to Dec 19)
Pnamenotu (Dec 20 to Jan 18)
Pharmuthi (Jan 19 to Feb 17)

Shemu (Season of the Harvest)
Pakhons (Feb 18 to Mar 19 (18*))
Paoni (Mar 20 (19*) to April 18 (17*))
Epep (April 19 (18*) to May 18 (17*))
Mesore (May 19 (18*) to June 15 (14*))

* Date on leap years.

More Egyptian religion and culture...

The calendar used in western civilization in fact is the Egyptian calendar with its 365 days in a year. The Egyptian calendar was introduced in an improved form by Julius Caesar in Rome in 46 b.C. Since a minor correction was added under Pope Gregory XIII in 1582, we now speak of the Gregorian calendar. Which in fact is too small an honour to the Egyptians...

The following is an adoption from http://serendipity.magnet.ch/hermetic/cal_stud/cal_art.htm:

Following his conquest of Egypt in 48 B.C. Julius Caesar consulted the Alexandrian astronomer Sosigenes about calendar reform. The calendar which Julius Caesar adopted in 46 BC was identical to the Alexandrian Aristarchus' calendar of 239 BC, and consisted of a solar year of twelve months and of 365 days with an extra day every fourth year. It is unclear as to where or how Aristarchus arrived at this calendar, but one may speculate that Babylonian science was involved.