Ancient months usually bear the name of the major festival. We are now in the ancient Athenian month of Poseideon. Poseidon as savior of ships, protector of those who voyage in ships, and God of the lapping waters both salt and fresh important for agriculture, is thanked for the many gifts that came from faraway places that were likely given at that time. The immense trade and distribution was nearly all through shipping, relatively little overland, whether it be perfume from Cyprus or pottery from Corinth. It is interesting today that Agios Nikolaos (Saint Nikolas) is the Patron Saint of seafarers in the Orthodox Church. Celebrating Poseidon's Festival seems to be lost in modern practice. It likely entailed bonfires, feasting, cutting of trees (probably decorated), and very likely gift giving. As God of begetting, that aspect was not forgotten. The most complete account of the festival is Noel Robertson's article Poseidon's Festival at the Winter Solstice, The Classical Quarterly, New Series, Vol. 34, No. 1. (1984), pp. 1-16:
"The record shows that Poseidon was once worshipped in every part of greece as a god of deneral importance to the community."
"The festival falls near the winter solstice, and the ritual business marked by jollity and license, belongs to the general type of solstice festival known the world over. At Poseidon’s festival, however, the sportive conduct has a definite purpose; this purpose arises from the fundamental agrarian background if Mediterranean society, and may bring us close to the origin of solstice festivals."
"It has scarcely been noticed that festivals of Poseidon, more than those of any other Greek deity, fall at just this time of year; yet the evidence is extensive."
"The festival Poseidea and some of the rites in question are often claimed for Poseidon the sea-god, but at this season sailing is furthest from one’s mind, and fishing on the shore is by no means an overriding concern. Such details as we have point elsewhere, to Poseidon as the god of fresh water who fructifies Demeter’s fields."
One of Poseidon’s epithets is prosklystios, 'of the lapping water'. He is also invoked as Poseidon phytalmios which implies natural fertility and human procreation. There are also implications in the legends that imply bonfires at the winter solstice.
Noel Robertson concludes:
"…the celebrants feast to satiety, then turn to lascivious teasing. What is the ritual purpose of such conduct? It obviously suits Poseidon’s mythical reputation as the most lustful of gods, who far surpasses Apollo and Zeus in the number of his liaisons and his offspring. Poseidon the seducer is the god of springs and rivers; his women typically succumb while bathing or drawing water; the type of the river god is a rampant bull. But the ritual likewise treats Poseidon as a procreant force; witness the epithets phytalmios, genesios, pater, etc. as interpreted above. The myths and the ritual reflect the same belief. The rushing waters are a proponent male power, just as the fields which they fertilize are a prolific female. Both water and the fields, both Poseidon and Demeter, can be made to operate by sympathetic magic. The rites of our winter festival rouse Poseidon and bring the rushing waters…"
It is interesting that that Theophrastus tells us the the silver fir was important in ship building, especially for masts. The ‘tannenbaum’ is a silver fir. It is also interesting to compare with the Roman Saturnalia which may very well have borrowed from the Poseidea.