Wikipedia
Bendidia or Bendideia was an ancient religious festival celebrated at Athens since 429 BC in honor of Bendis, a Thracian goddess whom the Greeks identified with Artemis.
Bendideia, festival in honor of Bendis- processions. “Perhaps we do not know that the Bendidia intend to worship Artemis according to the Thracian usage and that this name, Bendis, is Thracian? Thus also the Theologian from Thrace (Orpheus) among the many names of the Goddess Selene refers to Her also the name of Bendis: ‘Plutonide and Euphrosine and mighty Bendis.’ As for the Panathenaic festival, I mean the Lesser, which come after the Bendidia, and had as reason for the feast Athena. Well, the one and the other are the daughters of Zeus, both are virgins, then you add that both are 'bearers of light’, although Bendis as the one who brings to light the invisible principles of nature, while Athena as the one who gives intellectual light to the souls .. and also as the one who dispels the darkness, whose presence prevents souls to see what is the divine reality and what is the human. Now, since these are the characteristic properties of both, it is clear that Bendis is the guardian of becoming and presides over the births of the principles that belong to the becoming …” (Proklos, In RP. I, 19).
Sacrifice to Menedeios. (Sacrificial Calendar of Erchia)
The nineteenth is always dedicated to purifications and apotropaic rites. “The traditional laws of the Athenians have attributed the eighteenth as well as the nineteenth to the lustral and apotropaic rituals, as told by Philochorus and <***>, both interpreters of the uses of their Ancestors. So, perhaps for this reason, Hesiod says that this day is sacred, and especially after noon, because this part of the day is suitable for the purification's…” (Scholia Erga, 810).
Other times, Bendis is conflated with Persephone or Selene. These associations also have their similarities. Bendis’ ceremonies were indeed practiced at night, by Thracians and (after the Oracle of Dodona told them to) by Greeks as well. One such ceremony is mentioned as the setting for Plato’s Republic, and appears in other classical works. It is called the Bendidia.
Bendidia involved torchlight processions on horseback and revels under darkness, like those celebrated by other Thracian and Orphic deities like Sabazios and Dionysos. Bendis is sometimes accompanied, like Dionysos, by satyrs and maenads, or simply by athletes running toward Her temple.
The Goddess Bendis:
A Thracian mother goddess. Bendis was often identified with the huntress goddess, Artemis and the moon goddess, Hecate. She is also the goddess of healing.
Bendis first appeared in Athens, during the Peloponnesian War, where she was Hellenised. An annual feast, Bendidea, was held in her honour. The geographer Strabo says that the rites and customs of the Bendidea are like those found in Thracian and Phrygian types of revelry - Bacchus ( Dionysus) and Rhea (actually Cybele). The philosopher Plato also mentioned the festival in the dialogue between Adeimantus and Polemarchus of The Republic (Plato) about a horse-race at night, where each rider carried a torch.
Her attributes include Thracian-style pointed hat and boots made of fox-skin, while holding up a torch in one hand.