Synoikia

The Synoikien were an ancient Greek festival that was held in Athens to celebrate the political unification of former rival each other cities of Attica to the capital Athens.

According to Thucydides, the mythical Theseus was the one who determined the municipalities of Attica and Athens united to the capital. Before Theseus the various settlements of Attica were autonomous and in discord with each other. Theseus ended the autonomy of municipalities and therefore the skirmish and brought peace to Attica.

Plutarch dates the celebration on the 16th July and then tells you that even a so-called peace offering was made.