(19th Thargelion) This was a festival of the Thrakian goddess Bendis, whose cult was introduced to Athens in 429 BCE by Thrakians resident in the Peiraios at the time. Their shrine to her was built on the hill Mounykhia, near to the temple of Artemis, with whom She is identified. She is often pictured as a huntress, another characteristic she has in common with Artemis. Plato reported a bit about the first festival to Bendis at the beginning of The Republic. From his account, we learn that native Athenians took part and provided a procession as fine as that of the Thrakians, and that a torch race was to be held—and on horseback, which was a novelty—and an all-night festival. From other sources it’s known that the Athenian procession began at the Prytaneion in the morning and wound down to the sanctuary in the Peiraios, while the Thrakian procession was entirely within the Peiraios. The six-mile procession of the Athenians was so unusual, that a decree called for basins, water and sponges to bathe after it, and garlands. Then, presumably, a festival meal was provided. After an interval, perhaps a midday nap, the evening torch race and all-night festival was held. What function Bendis served is not described anywhere. Herodotos wrote that Thrakian women offered wheat to Her, and the Bendideia is held near to the time of the Attic grain harvest. But Thrake bordered the route by which Athens imported grain from the Black Sea area to feed its citizens, and there could be a connection there (from Parke 149-152).
Adkins and Adkins note that a sacrifice to the local hero Menedeios by the Attic deme Erchia was also held on this date (356).
Adkins, Lesley and Roy A. Adkins, Handbook to Life in Ancient Greece, 1997
Parke, H.W., Festivals of the Athenians, 1977