The Eleusinian Mysteries were initiation ceremonies held every year for the cult

Cult (religious practice)

In traditional usage, the cult of a religion, quite apart from its sacred writings , its theology or myths, or the personal faith of its believers, is the totality of external religious practice and observance, the neglect of which is the definition of impiety. Cult in this primary sense is...

 of Demeter

Demeter

In Greek mythology, Demeter is the goddess of the harvest, who presided over grains, the fertility of the earth, and the seasons . Her common surnames are Sito as the giver of food or corn/grain and Thesmophoros as a mark of the civilized existence of agricultural society...

 and Persephone

Persephone

In Greek mythology, Persephone , also called Kore , is the daughter of Zeus and the harvest-goddess Demeter, and queen of the underworld; she was abducted by Hades, the god-king of the underworld....

 based at Eleusis in ancient Greece

Ancient Greece

Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

. Of all the mysteries

Sacred Mysteries

The term sacred mysteries generally denotes the area of supernatural phenomena associated with a divinity or a religious ideology.-Pre-Christian religious mysteries:...

 celebrated in ancient times, these were held to be the ones of greatest importance. It is acknowledged that their basis was an old agrarian

Agrarianism

Agrarianism has two common meanings. The first meaning refers to a social philosophy or political philosophy which values rural society as superior to urban society, the independent farmer as superior to the paid worker, and sees farming as a way of life that can shape the ideal social values...

 cult which probably goes back to the Mycenean period

Mycenaean Greece

Mycenaean Greece was a cultural period of Bronze Age Greece taking its name from the archaeological site of Mycenae in northeastern Argolis, in the Peloponnese of southern Greece. Athens, Pylos, Thebes, and Tiryns are also important Mycenaean sites...

 (c.1600-1100 BC) and it is believed that the cult of Demeter was established in 1500 BC.