Modern American pagans often consider Poseidon in the summer, when they make their way to beaches across the land. However – in ancient Attica the month of Poseidon feel in mid-winter.
The Poseidea (festival of Poseidon) was celebrated over the period of time that corresponds to our modern December – January. It’s is curious to some that his festival would be held during the time the Greeks were least likely to set sail. However – prayers for the return of calmer seas and hence safer travel would be necessary for this culture that thrived on the shore. Winter in the Winter in the Mediterranean can bring gales, rain and hypothermia.
Because of its ancient roots, and where it falls in the modern calendar, the Poseidea is commonly celebrated today along with with the Winter Solstice. 16 days of feasting are traditional for the Poseidea celebration, with rites of Aphrodite concluding the festival.
“At Eleusis there was a festival called Haloea on the 26th of the month Poseideon. The Haloea, a festival for Demeter and Dionysus, included a procession for Poseidon. The Poseidonia of Aegina may have taken place in the same month, and included 16 days of feasting with rites of Aphrodite concluding the festival.” Source
Today, many Hellenic reconstructionists use the 8th Day of each month is honor to Poseidon as his sacred day. The day is marked with prayer and offerings to this mighty God of the Sea. Other appropriate times to honor Poseidon are those times of personal need, at the successful completion of actions, or when embarking upon a journey that would benefit from the Gods assistance.