I'll be honest--after my local Hellenic group exploded and I spent time recovering from the hell I went through being subjected to all of their shit, I stopped celebrating the Hellenic holidays. I put up an olive branch last year on my door--I think during the Pusnepsia for the eiresone--but that was essentially it. I've done my usual offerings and libations and what-not throughout the year, but the only real festivals I've done so far have been a solitary New Moon ritual for Hekate, a Thelemic version of the Samhain festival, the equinoxes, and a Festival of Hekate.
Hekate's REALLY been present these days--glad she's got an altar by the door of my bedroom now. :)
I'm not even certain what I would celebrate if I did. There are very few Athenian festivals that grab me: Anthesteria, Thargelia, Rural Dionysia maybe. I like the idea of honoring secular holidays and the solistices and equinoxes in a Hellenic fashion--I really don't want to create my own holidays out of thin air, and I think aspects of the Pagan wheel of the year make sense to me. At the very least I think that the solistices and equinoxes speak to me. The changing of the seasons always affects me on some profound level.
Festivals came about due to local customs and actual meaning, and were localized to a particular city or town. The winter time has always meant Dionysos to me, while the summer has always meant Apollo. The equinoxes have always made me think of Demeter and Persephone. So perhaps those would be good places to begin. New moons for Hekate, Samhain for Persephone and other chthonic deities, Beltaine for Aphrodite. I think full moons would be a great time to honor Artemis and/or Diana.
Cycles of nature are timeless, and I need Dionysos to keep me warm during the cold New England winters. Brr.