Sothis was the Egyptian name for the star Sirius. To the Ancient Egyptians Sirius and the sun were the two most important heavenly bodies. Together they marked the start of the year and the annual inundation of the Nile. (Plutarch called the Nile by the name Sirius.) This occurred when the star was seen on the horizon just prior to the rising of the sun in the east. It was also close to the time when the sun moved into the constellation of Leo.
Many centuries of observing this event enabled the architect Imhotep in 2686 BCE to plan the step pyramid at Saqqara with its entrance orientated towards this point. The Egyptians believed that it was the combined strength of the two heavenly bodies which caused the exceptionally hot weather that followed (now known as the Dog Days after the Dog Star, Sirius).
The Egyptians credited Thoth, the god of time, with the invention of the calendar. The year was made up of 3 seasons of 4 months. Each season was named after the main activity of that part of the year: the Flood, Planting and Harvest. Each month was made up of 3 weeks of 10 days each. A further 5 days (and 6 in every fourth year) completed the 3651/4 day year. (The days themselves were divided into 10 hours of 100 minutes each, divided again into 100 seconds. This gave 100,000 seconds from midday to midday.) The additional 5 days were the birthdays of Osiris, Isis, Set, Nephthys and Horus. The owner of the extra leap year day is unknown but it has been suggested that it was ascribed to Thoth, as the month which followed, the first month of the year, was the month of Thoth.
The first day of the year was also the birthday of Hathor (whose temple at Dendera was aligned on the star Sirius), and was marked by great celebrations. Feasts were also held on this day to celebrate mankind's escape from the wrath of Sekhmet. In the pyramid texts both Hathor and Sekhmet are linked with the star Sept, another name for Sirius.
As the New Year's Day brought the life-giving floods it symbolized the rebirth of the land and was considered to be most auspicious. The official crowning of kings was performed upon this day.
Isis was the main deity associated with Sirius, and she was known as Sothis and also 'the lady of the star'. Osiris however was represented by the constellation of Orion. One of his titles was 'lord of perfect black' and he was known as a black god - black being symbolic of regeneration. This explains the importance of the Egyptian new year, for it was then that Isis and her husband, newly restored to eternal life by her skill and the aid of Thoth, appeared in the sky to herald the birth of their son Horus, the god of the sun, and proclaim the coming of the life-giving waters of the Nile to regenerate the earth.
Sirius lies in the constellation known as Cannis Major, the great dog, and Osiris, like Anubis, his son and assistant in the underworld, was represented by the image of a dog.
(Some anthropologists working Northern Africa tell us that the Dogon tribe of Mali, reputedly descended from predynastic Egyptians, have preserved traditions relating to the stars Sirius A and its dark partner Sirius B - which is invisible to the human eye, and tell of visitors arriving from Sirius in the distant past. Theorist have concluded from this that the Ancient Egyptians must have known about the dark star, however, this, and the work of the original anthropologists is disputed by others in the field.)