Each year, when it was time for the crops to be sown, in the month of Boedromion (September), this was the time of year for the Mysteries to be held. It all stems from the myth of Demeter and Persephone, when Hades, took Persephone (Kore -"maiden") down into the underworld. Demeter searched the world looking for her daughter, and while she searched Demeter neglected her duties and let the earth go barren. The gods were worried and Zeus, who had witnessed the abduction, intervened. Before she went back to the world of the living, Hades gave Persephone a pomegranate to eat, thus she would always be connected to his realm and had to stay there one-third of the year. This symbolic death and rebirth is the time the seed lies in the earth and then comes to life, reborn, as was Persephone. This was the basis of the cult, a fulfilling and happy afterlife.
The ceremony began in Athens, and all those participating purified themselves by bathing in the sea, they also sacrificed a piglet. All sacred and secret objects that were to be used in the ceremonies, were kept in the Eleusinion (a temple located at the foot of the Acropolis in Athens). These objects had been brought from Eleusis some days prior to the start of the celebration. It was from the Kerameikos (the ancient cemetery of Athens) that the great procession of initiates started. The "Mysteries" were given this name (in Greek musteriai, from mustes, ("an initiate"). As the procession proceeded on route to Eleusis the participants would, at a certain place, shout obscenities. This was a re-enactment of an old mythical woman called "Iambe" who was said to have made Demeter smile, at a time when she was full of sorrow for the loss of her daughter Persephone. Also during the procession their would be shouts of Iakch' o Iakche, which was related to an equivocal deity, referring to "Iacchus" who was identified with Dionysus, but in some versions said to be the son of Persephone or Demeter.
When the procession reached Eleusis they would rest and make ready for the next day, which was a day of fasting (Demeter did this when in mourning for Persephone). Once this part of the ceremony was over, the initiates drank a special brew of barley water mixed with penny-royal called, kykeon (a creeping mint cultivated for its supposed medicinal properties). The climax of the ceremony took place in the "Telesterion" (initiation hall). During the 5th century BCE, "Ictinos" designed a huge hall which would hold several thousand people. In this hall, the secret and sacred objects were shown to the initiated, and also the priestesses would reveal the vision of the holy night, which is thought to have been a fire symbolizing life after death. These rituals were kept secret, shown only to the initiated, and it was totally forbidden to speak of them publicly.
In the Hellenistic age (300-150 BCE), the cult was taken over and run by the state, and two aristocratic families from Eleusis officiated (the Eumolpidae and Kerykes). In this age, mystery cults were becoming very popular, unlike classical Greece (400s BCE) when the Eleusinian mysteries were a rare form of worship. The annual Eleusinian mysteries attracted thousands of people from all over the Greek world, and the only initial requirement to become a mystes (initiate) was to be without blood guilt nor a barbarian ( in other words, if you spoke Greek). It was open to both men and women, and remarkably, slaves were also allowed into the cult. The mysteries existed from Mycenaean times (circa 1600-1200 BCE), thought to have been established in the 1500s BCE and held annually for two thousand years. The Roman emperor Theodosius closed the sanctuary in CE 392, and finally it was abandoned when Alaric, king of the Goths, invaded Greece in CE 396. This brought Christianity to the region, and all cult worship was forbidden.