The Thracians had a moon goddess, called Bendis, a very wild sort of goddess. Due to an oracle given at Dodona, the Athenians had established a shrine for her. Both the Athenians and Thracians had festivities devoted to her, and in the fifth century these festivities, while still remaining distinct, had become so popular that the day was made an official holiday, the Bendideia. While Thracians and Athenians had distinct processions, the festival was given full ceremonial backing by the state. In the beginning of the Republic Socrates tells us has gone down to the Piraeus with Glaucon for the very first such holiday, to see how it would be celebrated.
" I was delighted with the procession of the inhabitants," he says; "but that of the Thracians was equally, if not more, beautiful." After seeing the processions and making their prayers, Socrates and Glaucon begin to head back to Athens, but are stopped almost immediately when young Polemarchus has a servant physically grab Socrates's cloak (not the last time in the dialogue that Polemarchus will stop someone with a grab). Polemarchus is at the festival with a group of young men, including Adeimantus and Niceratus. Polemarchus insists that Socrates and Glaucon remain -- indeed, he jokingly threatens to force them to stay because they are outnumbered. When Socrates still refuses, Adeimantus points out that there are evening festivities -- especially a horseback relay race with torches. And Polemarchus dangles the last bit of bait by saying that a whole group of young people are going to be getting together to talk, so Socrates and Glaucon stay, and the rest of the dialogue occurs in Polemarchus's house.
Commentators have occasionally speculated as to the significance of this elaborate set-up. Descending and ascending end up being important concepts throughout the dialogue, most strikingly in the Allegory of the Cave, and sosome have suggested that there is an important significance to the fact that Socrates and Glaucon descend to the Piraeus, and this seems to be plausible. Others have suggested that there is an irony, since Socrates will be charged later with worshipping foreign gods, that the whole scene is a festival in which a foreign god is given official recognition. Others have suggested a sharp contrast between the wild revelry of the barbarian festival and the civilized topic of the discussion.
There is probably something to all this, although it's hard to know how far to take it. But I would suggest that, at the very least, there is something else going on. I mentioned that Socrates says that the procession of the Thracians is just as beautiful as the procession of the Athenians, if not more so, and that Bendis was a Thracian goddess. The Thracians are mentioned explicitly one more time in the dialogue:
Must we not acknowledge, I said, that in each of us there are the same principles and habits which there are in the State; and that from the individual they pass into the State?--how else can they come there? Take the quality of passion or spirit; it would be ridiculous to imagine that this quality, when found in States, is not derived from the individuals who are supposed to possess it, e.g., the Thracians, Scythians, and in general the Northern nations; and the same may be said of the love of knowledge, which is the special characteristic of our part of the world, or of the love of money, which may, with equal truth, be attributed to the Phoenicians and Egyptians.
Notice that the Thracians are associated with "the quality of passion or spirit" while "the love of knowledge" is associated with the Athenians. The love of knowledge, of course, is closely associated with reason. And the quality of passion or spirit here is thumos. It's hard to translate; 'spirit' in the sense of 'spirited' is very close, which is why it is usually used, but it's our drive for exaltation, honor, eminence, glory. Thumos is important in the Republic, and what is notable is that much of the dialogue is concerned with arguing that justice in human beings requires a particular relationship between thumos and reason. One sees this, for instance, in Socrates's many-headed image of man: we have a human head, a lion head, and a many-headed monster. The human head is our reason; the heads of the many-headed monster are our passions; and the lion head is our thumos. Left to themselves the heads of the many-headed monster will terrorize the human head and, because they have no unity will drive a human being every which-way. But if the human head and the lion head work together, they can intimidate the many-headed monster, giving it unity and order. Other examples of the important relation between reason and thumos can be found.
It's significant, then, that the Thracians, who are associated with thumos, and the Athenians, who are associated with reason, are joined together in the festival: they remain distinct, but they both play a part. And, remember, Socrates said that the procession of the Thracians was a part of the festival just as beautiful as that of the Athenians. The Piraeus, with its heavy mixture of Athenians and Thracians, on the Bendideia, the day that is most due to the mixing of Athenian and Thracian culture, is the perfect setting for a dialogue in which the relation between thumos, represented by the Thracians, and reason, represented by the Athenians, plays a key role. Even if we read a sort of implicit disapproval of the revelry and foreignness of the festival (I think it, like Plato's disapproval of democracy, is often exaggerated), it is surely an ironic disapproval, because the foreign revelry is itself a sort of picture of what the Republic proposes. From the very beginning, the very way in which the dialogue is set, we know that the what the Platonic Socrates is proposing here is a philosophical Bendideia, the idealized truth of which the Bendideia itself is a crude picture.