The year of the astronomical Egyptian Calendar began on the summer solstice, SS, when the Nile flooded. This system is thought to have begun ~ 3000 BCE (2750 BCE). The solar calendar was not modified by leap years so that it gradually shifted away from the New Year Day being on the SS day. The period of this cycle is ~1500 of our years, when New Year once again began on SS. It is highly likely that the Egyptians, being fully aware of this drift, deliberately used it for a higher order calendar cycle. For example, by the time of Khufu of the Fourth Dynasty, the New Year Day was already between the SS and the spring equinox.
The estimated date of the start of the calendar is based on the heliacal rising of Sirius on the SS. In other words, when both the Sun and Sirius rose in the eastern sky, and which signaled the beginning of the flood season. After a drift of about 1500 years, on about 1500 BCE (1250 BCE), the New Year would again fall on the SS. This did happen at the time of Ramses II.
Although the Egyptian religion venerated the Sun, the astronomers were keenly interested in the rising of stars, thus they had a sidereal calendar. In particular, they watched for the heliacal rising of the Orion constellation which symbolized Osiris, and for the star Sirius (Sothis in Greek) which represented Osiris’ consort Isis. The star knowledge of Egypt may have come from Babylonia. In any case, the sidereal calendar was based on the heliacal rising of Sirius/Sothis.
The ritual calendar of the astronomer-priests was thus the Sothic or Sidereal year of 366 risings of Sirius. On the other hand, the agricultural solar calendar of the farmers was attuned to the summer solstice when the Nile flooded its banks and planting could begin.
Note: The above chart is from Belmonte, http://www.iac.es/proyecto/arqueoastronomia/pages/project-overview/highlights-hitos/egyptian-calendar.php
Labels in red have been added to indicate the times of Khufu (IV Dynasty), V Dynasty, Middle Kingdom, and Ramses II.
Egypt Code by Robert Bauval, 2010, provides further information about the Egyptian calendar, in particular the sidereal calendar.