"The Thesmophoria was one of the most widespread ancient Greek festivals. The fact that it was celebrated across the Greek world suggests that it dates back to before the Greek settlement in Ionia in the eleventh century BCE. The best evidence for the Thesmophoria concern its practice in Athens, but there is also information from elsewhere in the Greek world, including Sicily and Eretria.
The festival was dedicated to Demeter and her daughter Persephone and was celebrated in order to promote fertility, both human and agricultural. It was celebrated only by women, and men were forbidden to see or hear about the rites. It is not certain whether all free women celebrated the Thesmophoria, or whether this was restricted to aristocratic women; whichever was the case, non-citizen and unmarried women appear not to have celebrated the festival. In fact, participation was expected of all Attic wives, and could serve as a form of proof of marriage.
In Athens, the Thesmophoria took place over three days, from the eleventh to the thirteenth of Pyanepsion. This corresponds to late October in the Gregorian calendar, and was the time of the Greek year when seeds were sown. The Thesmophoria may have taken place in this month in other cities, though in some places – for instance Delos and Thebes – the festival seems to have taken place in the summer, and been associated with the harvest, instead. In other places the festival lasted for longer – in Syracuse, Sicily, the Thesmophoria was a ten-day long event.
The main source about the rituals of the Thesmophoria comes from a scholion on Lucian's Dialogues of the Courtesans. A second major source is Aristophanes' play Thesmophoriazusae; however, Aristophanes' portrayal of the festival mixes authentically Thesmophoric elements with elements from other Greek religious practice, especially the worship of Dionysus."