The Ancient Egyptian Calendar centered around the star, Sirius.
Sirius is part of the constellation Canis Major, which looks
like a dog in the sky. Sirius takes the form of the dog's nose
in that constellation. In the night sky, Sirius is located to
the lower left of Orion.
Canis Major could first be seen in the sky around June 21st of
every year; the Egyptians called this "the going up of the
goddess Sothis." The star was visible just before sunrise
and it is still one of the brightest stars in the sky.
The reappearance of the star Sirius in the sky was particularly important
to the Egyptians because it told them that the Nile river would soon begin
to flood, bringing water and rich new soil to the dry land where they
would plant their crops.
Years of careful observation of the sky and the river had taught
the Egyptians that the waters started rising around the end of
June, just after Sirius can be spotted in the sky. The flood period
lasted until October, covering the land with rich black mud and
preparing it for the sowing and growing period. The harvest time
started at the end of February and ended with the new Nile flood.
This predictable, ongoing cycle defined the agricultural year.
The Egyptian Calendar is also based on the lunar cycles as their
year had 12 months with 30 days each. This only adds up to 360
days in the year, but the Egyptians had an extra five days in
addition to the 360 calendar days at the beginning of every year
on which they celebrated.