The market of Kato Patissia is located in Odos Zacharia Papantoniou (follow Acharnon after Kato Patissia, it's on the left side after Kaftanzoglou).
It consists of an outside market for fruit, vegetables, eggs, and fish, as we expect it the market is noisy, messy, colorful and an experience for the nostrils.
This market is open only Thursday morning.
For more informations about the markets (Laiki) in Athens and their opening day, have a look at: AngloInfo