The calendar used by the ancient Greeks—particularly the Athenians—was strikingly different from the one we follow today. Rooted in the lunar cycle, their system closely followed the phases of the moon, resulting in a lunisolar calendar where each month lasted approximately 29.5 days.

Because of this lunar structure, the twelve months of the year weren’t enough to keep the calendar aligned with the solar year and the seasons. To resolve this, the Greeks periodically added an extra, intercalary 13th month, helping to maintain seasonal consistency and prevent drift over time.

A Calendar of the Polis

Each city-state (polis) in ancient Greece had its own calendar system and month names. However, the Athenian calendar is the best documented and widely studied. The months in this calendar were often named after religious festivals or deities associated with the period in which the month fell.

Here are the 12 standard Athenian months:

Hekatombaion
Metageitnion
Boedromion
Pyanepsion
Maimakterion
Poseideon
Gamelion
Anthesterion
Elaphebolion
Mounichion
Thargelion
Skirophorion

The 13th Month: Poseideon II

When the calendar needed to be realigned with the solar year, the month of Poseideon was often doubled. This gave rise to a 13th month known as Poseideon II or Second Poseideon. This intercalation typically occurred every three years, depending on astronomical observations and civic needs.

Time and Civic Order

Remarkably, the Athenian calendar wasn’t just a way to measure time—it also structured political life. The year was divided among the twelve tribes (phylai) of Athens, each named after a legendary hero. These tribes not only represented the population but also took turns holding various administrative and civic responsibilities month by month.

More Than a Timekeeping Tool

For the ancient Greeks, especially the Athenians, the calendar was far more than a schedule of days—it was deeply intertwined with religion, governance, and daily life. From planning sacred festivals to managing civic duties, their calendar reflected the complex rhythms of their culture and cosmos.