The Haloa is part of the Mysteries, and thus linked to the festivals of Proerosia (6 Pyanepsion), Thesmophoria (11-13 Pyanepsion), the Lesser Mysteries (20-26 Anthesterion), Stenia (9 Pyanepsion), Skirophoria (12 Skirophorion) and the Greater Mysteries themselves, which were held 13-23 Boedromion. It was a rural festival, meaning it wasn't state-organized and widely spread, so most details are incredibly fuzzy. He're what we do know about it:
The Haloa is assumed to be a celebration of the pruning of the vines and the tasting of the wine after its first fermentation, or it may be to encourage the growth of corn from the seed. It is named after the hálōs (ἅλως), which means both threshing floor and garden. Since the first sense of the word would be inapplicable to a festival celebrated in January, scholars--including Nilsson in his 'Greek Popular Religion'--insist it must have been a gardening festival.
Some time during the festival, the entire population was invited by the priests of Dionysos and the priestesses of Demeter and Kore to give sacrifice to these Theos; to Demeter and Kore for the fertility of the earth in which the grapevines grew, and to Dionysos in remembrance of Ikários, who was such a fine winemaker that he could produce wine so strong, those who drank it appeared to be poisoned. His skill turned out to be his undoing; Íkaros was killed by those who drank his wine, thinking the wine maker was out to kill them. This sense of being poisoned might have come from permanent erections brought on by the wine; they only went away when an oracle told the men to put out clay phallus-shaped objects. When they did so, their limpness returned. This event was celebrated at home, by the men, but the women traveled to Eleusis.
In the earliest times the religious part of the festival might have been restricted to married women, but after the fourth century BCE its celebration may have been limited to hetairai (ἑταῖραι, female companions, a term used non-sexually for women, about women, but used by men to indicate a woman hired for entertainment, often leading to sex), or they were simply also allowed to hold their own symposium during the Haloa, either at home or at Eleusis. The Haloa would have been the day on which they were initiated into the Mysteries. The Eleusinian Arkhontes (Ἄρχοντες, male magistrates of the Mysteries) prepared a huge banquet on this day, with a huge variety, including phallus- and vagina-shaped cakes, but not foods forbidden in the Mysteries: pomegranates, apples, eggs, fowls, some types of fish were out. Animal sacrifice was also disallowed on this day: Demeter received offerings of fresh fruit.
After preparing the food, the Arkhontes left, leaving the women to eat, to drink lots and lots of wine, and to celebrate being a woman and fertile (or the wish to be fertile). The Arkhontes went to the men who were waiting outside of Eleusis for their part in the Mysteries, and told them the story of Eleusis, and how the Eleusinians had discovered nourishment for the entire human race. A giant phallus is often assumed to have been set up on the hálōs, and the women would dance around it carrying clay models of phalli and vaginas, but it is more likely the phallus was never there, but depicted on artwork abbot the Haloa to indicate the fertility aspects of the festival and the dances that occurred there. As part of the festivities, the women engaged in sexualized conversation with each other. As part of the sacrifice, the women carried kernoi (κέρνοι, offering dishes) on their heads, containing incense, grains or other offerings, which they tipped onto the giant phallus or, and this is probably far more accurate, onto the altar.
After the feast and sacrifice, the men who had been waiting were admitted to the grounds, and the women were encouraged by each other--including the priestesses--to take secret lovers for the night. A priest and priestess--with torches representing Demeter and Persephone--apparently sat watch on chests as they presided over the fertility celebration.
The Haloa is the perfect time to organize an adult 'girl's night' with your closest female friends. Watch a movie with erotic tones, drink wine together, gorge yourselves on chocolate and gossip about your partners. If you have an agreement about it with your partner, you could find a lover for the night. If not, go home to him or her and spend the night together in your own 'fertility rite'. If you're single and have no one to fill your bed... well... a girl can get creative, can't she?
For the men, the Haloa might have had an extra ritual part as well; honoring Poseidon as an agricultural Theos. There is evidence that the men built a huge bonfire and had their own conversations around it. Afterwards, they joined the women, when possible (and desired). Single men; I'm sure you can be as creative as the single women reading this.
Enjoy your Haloa celebration! Be safe, happy, and sated!