Today is about Festivals, Days, and times sacred to Poseidon. This is one of those posts where I’m going to remind people that the Attic (Athenian) Festival Calendar, which many modern Hellenic Polytheists use for the timing of festivals, was lunisolar. This means that although we are used to a more “fixed” calendar like the Gregorian calendar that most of the world uses at this time, the Attic Festival calendar was in many ways similar to the Jewish calendar. This means that the festivals will not be on the same day each year in our civic calendar, but will always be the same day on the religious one.
In the Classical period, it would seem that the eighth day of the lunar month was sacred to Poseidon so this would be something that we should be aware of if we devote a significant cultus to Him. It looks like there was only one “formal” festival honoring Poseidon in the Classical period. It was the Poseidea and will sometimes be as early as happening in November or as late as in December. There is little known about this festival from the Classic writers, so it is not one that would necessarily be “easy” for us to re-create.
However, Poseidon is unique among the Hellenic Theoi to have a month (sometimes two) each year named for him due to it being named after the Poseidea festival. The reason that it is sometimes two months in the Attic calendar instead of one is because of the calendar being lunisolar. Like the Jewish calendar, every few years a “additional” month is added in to maintain the calendar’s relative synchronicity with the solar year. In the Attic calendar, this month was added in winter and was done by a “doubling” of the month Poseideon.