Although only one festival of this name is attested and that in the region of Marathon, it does seem likely, given the existence of a month called Poseidion, that at one time a major festival of that name existed in Attica. Since the 8th of each month is dedicated to Poseidon, the best guess is that such a festival would have been held on 8 Poseidion
I’ve posted my ritual for the Poseidea at my other blog.
A PDF version of the ritual script is available here.
