The ancient civil Egyptian calendar had a year that was 365 days long and was divided into 12 months of 30 days each, plus five extra days (epagomenae, from Greek ἐπαγόμεναι) at the end of the year. The months were divided into three weeks of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events therefore "wandered" through the calendar, it has been referred to as the annus vagus, or "wandering year".

Based on his understanding of the Palermo Stone, Alexander Scharff believed that the Old Kingdom period observed a year with 320 days.

A tablet from the reign of First Dynasty King Djer (c. 3000 BC) was conjectured by early Egyptologists to indicate that the Egyptians had already established a link between the heliacal rising of Sirius (Egyptian Sopdet, Greek Σείριος Seirios) and the beginning of the year. However, more recent analysis of the pictorial scene on this tablet has questioned whether it actually refers to Sothis at all. Current knowledge of this period remains a matter more of speculation than of established fact.