Egypt had three different calendars. The first, civil one was linked to the flooding of the Nile. It was divided into Nile flood, fertile period and time of drought. These three seasons were subdivided into four months with 30 days each; the five days prior to the onset of the New Year were added to that, and that made 365 days in total.

The Egyptian astronomers, however, had realized from time immemorial that the civic calendar was out of step with the astronomical one. For the latter, they took the rise of Sirius, Egyptian Sotis, as fixpoint. It took 364 1/4 days, which meant that after four years the civic calendar differed from the astronomical year for one entire day. It took 1461 years for both calendars to become synchronized again.

To read the complete article, see:Egypt and Alexandria. A brief numismatic survey: part III(www.coinsweekly.com/en/Egypt-and-Alexandria-A-brief-numismatic-survey-part-III/8?&id=340&type=a)