On the 15th and 16th of Hekatombaion, the Synoikia (συνοίκια or συνοικέσια) festival was held in Athens. It was a community festival, sacred to primarily Athena, and was somewhat of a two-day festival held every year. Why the Sunoikia was celebrated, and what its origins are is not entirely clear; best I can tell is that it reaches back to the unification of twelve small towns into the metropolis of Athens, and is thus linked to the myth of Theseus. The festival started last night and will run until sundown the 14th this year.

Above, I stated that the Synoikia was 'somewhat' of a two-day festival; the 16th was the official sacred day, but the 15th was important as well. Parke, in 'Festivals of the Athenians' (1977), states that:

"Some light on the subject comes from a fragment of the fifth-century code of sacrificial regulations found in the Agora. It records among the festival held every second year as the earliest in the calendar sacrifices held on the 15th and 16th of Hecatombaion. This is evidently the Synoikia though the name does not appear in the inscription. Thuclydides did not mention anything about a two-yearly celebration, and one would naturally expect the commemoration of a historic even to take place annually. But the part of the code dealing with the annual festivals of Hecatombaion is lost, and it probably contained references to the annual Synoikia on the 16th, and one should picture the celebration as taking place on this one day every year, and every second year being held in a larger and more extended form over the two days of the 15th and the 16th." [p. 31]

The second day was the main event, and it contained sacrifices to Zeus Phratrios, Eirênê (Ειρηνη, Goddess of peace and spring), and most importantly: Athena. The Synoikia was believed to have been instituted by Theseus to commemorate the concentration, the Synoecism, of the government of the various towns of Attica and Athens. This unification is described by Thucydides, in his 'History of the Peloponnesian War':

"In this manner spake the Mytilenaeans. And the Lacedaemonians and their confederates, when they had heard and allowed their reasons, decreed not only a league with the Lesbians but also again to make an invasion into Attica. And to that purpose the Lacedaemonians appointed their confederates there present to make as much speed as they could with two parts of their forces into the isthmus; and they themselves being first there prepared engines in the isthmus for the drawing up of galleys, with intention to carry the navy from Corinth to the other sea that lieth towards Athens, and to set upon them both by sea and land. [2] And these things diligently did they. But the rest of the confederates assembled but slowly, being busied in the gathering in of their fruits and weary of warfare." [3. 15]

Prior to this mythical event taking place, it seems the Synoikia was solely a festival for Athena, as caretaker of Athens. All sacrifices went to Her. After the Synoecism, however, Zeus Phatrios gained importance: he oversaw the various phratries (clans) of Athens who had come together to form a unified people. The content of the Synoikia was solidified in a time of many wars, and it seemed many people were not only tired of them, but saw them as a threat to the solidity of Athens and Attica. As such, the inclusion Eirênê makes sense.

Even in ancient times, the sacrifices were a bit lacklustre: a young ewe on the 15th, and two young bullocks on the 16th. Neither sacrifice included a feast and the meat--save for what was sacrificed, of course--was sold right away, indicating not many people attended and that the festival was held most for form; and antiquated festival even then. Today, reading up on the history of Athens and sacrificing to Athena, Zeus Phatrios, and Eirênê suffices to celebrate the Synoikia.