(15-16 Hekatombaion) Yet another holiday occurring during the important first month of the Athenian year that served to remind people of their history and origins, the Synoikia, literally “houses together,” was an ancient observance commemorating the unification of Attica by Theseus. The festival lasted perhaps one or two days on alternate years. Parke thinks it might have occurred annually, with a one-day festival on the first year and a larger two-day festival on the second year (31). Athena, as the patroness of the unified territory, was honored on the Synoikia, as was Zeus Phratrios, who oversaw the various phratries (clans) of Athens. A sacrifice made by the Athenians in late classical times to Eirene (Peace) may have been scheduled to occur at the same time as the Synoikia, which was a holiday likely celebrated devoutly by certain aristocratic families within Attica (Parke 32, 188).
As even Thucydides considered Synoikia to be ancient and not widely celebrated, it is enough to merely study the history of Attica on this day, and learn what is known of the unification of the several smaller communities into the larger civic region known as “Attica”. This may have happened in Mycenaean times or some time after that, in an era not well remembered in the Classical Era (Parke 31-2).
Perhaps, in this age of devastating global wars, it is more worthy to celebrate Eirēnē, the Goddess Eirene, in any way appropriate to your community. The Homeric Hymns 11 and 28 to Athena and the Orphic Hymns 32 to Athena and 33 to Nikē are appropriate to read.
Walter Burkert, Greek Religion, 1977, English version 1985
Parke, H.W., Festivals of the Athenians, 1977