https://www.newworldencyclopedia.org/entry/Dionysus
In Athens and the Attica of the Classical period, the main festivities were held in the month of Elaphebolion (around the time of the Spring Equinox) where the Greater, or City, Dionysia had evolved into a great drama festival - Dionysos having become the god of acting, music and poetic inspiration for the Athenians - as well as an urban carnival or Komos. Its older precursor had been demoted to the Lesser, or Rural, Dionysia, though preserved more ancient customs centred on a celebration of the first wine. This festival was timed to coincide with the "clearing of the wine," a final stage in the fermentation process occurring in the first cold snap after the Winter Solstice, when it was declared Dionysos was reborn. This event was later explicitly set to January 6, a day on which water was also turned to wine by Dionysos in a separate myth. The festivals at this time were much wilder too, as were the festivities of the grape harvest, and its "carnival-esque" ritual processions from the vineyards to the wine press, which had occurred earlier in the autumn. It was at these times that initiations into the Mysteries were probably originally held.
In sharp contrast to the daytime festivities of the Athenian Dionysia were the biennial nocturnal rites of the Tristeria, held on Mount Parnassus in the Winter. These celebrated the emergence of Dionysos from the underworld with wild orgies in the mountains. The first day of which was presided over by the Maenads, in their state of Mainomenos ("madness") in which an extreme atavistic state was achieved, during which animals were hunted - and, in some lurid tales, even human beings - before being torn apart with bare hands and eaten raw (this being the infamous Sparagmos, said to have been once associated with goat sacrifice, marking the harvesting and trampling of the vine). The second day saw the Bacchic Nymphs in their Thyiadic ("raving") state, a more sensual and benign Bacchanal assisted by satyrs, though still orgiastic. The mythographers would explain this with claims that the Maenads, or wild women, were the resisters of the Bacchic urge, sent mad, while the Thyiades, or ravers, had accepted the Dionysiac ecstasy and kept their sanity.