The Haloa

A winter festival in Attica for the fertility of fields and women, celebrated by women for women, focused on Demeter (as daimon of the crops), Poseidon (as provider of nourishing water), and Dionysos (as provider of wine). This is an example of how a festival could have overlapped multiple deities around common points of their domains. It also is an example of a festival that served a social function, in this case to generate camaraderie among the women.
Celebrated in Athens area in Eleusis, Attica, perhaps associated with the Eleusinian Mysteries. Much speculation has come from reconstruction based on spotty textual and pictorial evidence but it seems that there was a (possibly bloodless?) sacrifice, much feasting, crude gossip, and the consumption of erotic cakes in an all-night celebration. Held on the 26th Day of Poseidon's Month(?)
The overall intent may have been to "wake up" Demeter from her annual despondency over the seasonal absence of her daughter. It may also have been an annual reenactment of the myth, where Demeter is cheered up, then tries to burn away the mortality of Demophoon as a substitute child. There may also have been singing, dancing, and games around a central bonfire on a threshing floor.