The Hellenic calendar or more properly the Hellenic calendars— for there was no standardized uniformity imposed upon all of Classical Greece— began soon after the June solstice, or more specifically the time when the star Sirius rose just after the moment of dawn, its heliacal rising. The star was invisible at the moment, in the brilliance of the sun's light, so it took an astronomer's reckoning to establish the moment of the new year. According to Hipparchus, a Greek astronomer of the 2nd century BCE, Sirius rose with the sun at the latitude of Rhodes on the 19th of July, about a month following the summer solstice.
As Karl Kerenyi points out (Kerenyi 1976, pp 29ff), the onset of the fiercest killing heat of summer is a counter-intuitive beginning point for the Greek calendar. In Egypt, however, the calendar year, marked with the summer rising of the Nile, begins with the rising of Sirius ("Sothis" in the Egyptian calendar). Calendar systems are always part of the deepest embedded layers of culture, and Kerenyi remarks ""The connecting link could only have been the Minoan culture," where the Greek archaeologist Spyridon Marinatos connected the orientation of Minoan palaces with the sumnmer rising of Sirius.
Leading religious and political sites on the Hellenic mainland began their calendar with the rising of Sirius: Olympia, Delphi, Athens (see Attic calendar), Epidauros, and other Greek city-states with Mycenaean origins.
Reference
- Kerenyi, Karl, Dionysus: Archetypal Image of Indestructible Life (Bollingen, Princeton) 1976.