Final answer:
The Festival of Dionysus, or Bacchanalia, occurred biennially in ancient Greece with its timing varying by location, but in Athens, it was mainly celebrated in March/April during Elaphebolion. These festivals, including the Eleusinian Mysteries related to Demeter and Persephone in Eleusis, lasted several days and involved secret initiatic rites.
Step-by-step explanation:
The Festival of Dionysus, also known as the Bacchanalia, was traditionally held in ancient Greece and celebrated the god of wine, theater, and fertility. Despite the original festival's ancient time frame, references suggest that Dionysiac festivals were biennial and may have taken place in various locations during different times of the year. In Athens, the most famous Dionysian festival was the City Dionysia, which mainly took place during the month of Elaphebolion, roughly corresponding to March/April, signaling the start of the local sailing and farming season.
Additionally, The Bacchae by Euripides indicates that such celebrations were a common periodic event, although the exact reasons for a biennial period are not explicitly clear, it is noted that this may have connection to rites of initiation for young maidens reaching puberty. The festivities could vary in duration, with evidence indicating that the celebration could extend over several days. In Eleusis, the related Eleusinian Mysteries were held annually, starting on the 14th and climaxing on the 19th of Boedromion (September/October), and were known for their secrecy, as many of the initiatic rites within these mysteries were meant to be kept hidden from the uninitiated.