As mentioned in the introduction Lenaia and Anthesteria* are two major Athenian festivals dedicated to Dionysos. There is also the Dionysia, divided between Rural (Later winter months) and City (after Anthesteria). Usually the Dionysia would spread throughout Greece, during the spring and summer months. Surrounding towns would celebrate based on their local customs and time. Most often these festivals involved public performance, pomp / processions, plays, coming to age rituals, day of the dead, public drunkenness, vulgar language / insults, feasting, role exchanges between masters and slaves, prisoner pardoning, wine miracles, singing, sacrifice and religious observances and Mystery performance / initiation.
*Lenaia is celebrated between January to February. Anthesteria in February to March. Dates depends on the lunar cycle, likewise for other Hellenic festive dates.
In Italy these festivals are the Bacchanalia (The Dionysia) and the Liberalia (In part a indigenous holiday heavily influenced by Dionysian festivals from Greece).
Nowadays modern Polytheists still practice these rituals, although to a lesser extent to what is recorded in ancient times. Also some of these customs still exist in Greece, between March and May, with phallic processions or dressing in goat skins ringing bells, performed by locals who generally don’t identify as polytheist / pagan.
In my own practice I rarely celebrate festivals. This is in part my location being in the southern hemisphere and festivals never ‘jived’ with my personal practice.