The Attic Calendar was in use in Athens and its surrounding territories during its most productive period as far as classical literature is concerned (5th & 4th centuries B.C.E.). Although there were several other calendars in use at the time, and at different times throughout history, this one has the most documentation as it was used to run the internal affairs of the city. It is also the one many modern Hellenic Pagans choose to base their liturgical calendar on.
The calendar had 12 months and a 13th intercalary month had to be added every few years or so to keep the months in line with the seasons. This month was, at different times, put between different months throughout the calendar and was given the same name as the month that preceeded it. That is, every few years, some month would happen twice. It seems that Poseideon most often had the honor, but it's unclear
While each month within the calendar held its own festivals, certain sabbath days were observed each month.
The following is the generally accepted order of the months:
More information online
The Hellenion Liturgical Calendar at Hellenion.org
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