The ancient Egyptians had just one calendar in operation, the civil one, during most of their history and before the overwhelming influence of Hellenic culture. This calendar may have been invented for a specific purpose in the first half of the third millennium B.C., when the previous local Nile-based lunar calendars were rendered useless, as the result of the unification of the country and new social, economic and administrative requirements. The civil calendar always started at the feast of Wepet Renpet in the first day of the first month of the Inundation season (I Akhet 1). Its peculiar length of only 365 days (without leap years) might have been established from simple astronomical (presumably solar) observations. Consequently, Wepet Renpet wandered throughout the seasons in a period close to 15 centuries. Our research has shown that this phenomenology was reflected in the Egyptian worldview by the orientation of most important sacred structures accordingly.