Heortai | Festivals
"And they had no sure sign either of winter, or of flowery spring,
- Prometheus Bound, Aeschylus (trans. Buckley)
Most of our information surviving today details the Athenian (Attic) calendar as the festivals in Athens are the most well documented, and the Attic deme of Erkhia's calendar is the only complete calendar surviving. Unfortunately, not much information on the sacred and civil calendars of the other city-states has survived. Almost all of our calendars are incomplete and more still only persist as fragments.
The Ancient Greek calendars all follow a lunisolar pattern, meaning they're tracked by solar events such as equinoxes and solstices, as well as the moon phases. This likely descended from the Mycenaean calendar and persisted through the Greek Dark Ages up until Roman times when significant alterations were finally made in an effort to standardize.
A reconstruction guide on how the Ancient Athenian Calendar counts the days, months, and years
Attic Festival Calendar: Greek Athletic Sports and Festivals - Flickr Commons
Calendar Explained: Crescent Moon in Blue Sky - Photo by Lynn Greyling
Monthly Sacred Days: Sunrise Seen from Ferry in Port Kissamos - Photo by Alexey Komarov
Annual Festivals & Sacrifices: Dedication to Bacchus by Lawrence Alma-Tadema