According to Mylonas, the lesser mysteries were held “as a rule once a year in the early spring in the month of flowers, the Anthesterion,” while “the Greater Mysteries were held once a year and every fourth year they were celebrated with special splendor in what was known as the penteteris.” 
Kerenyi concurs with this assessment: “The Lesser Mysteries were held at Agrai in the month of Anthesterion, our February… The initiates were not even admitted to the epopteia [Greater Mysteries] in the same year, but only in September of the following year.” 
This cycle continued for about two millennia. In the Homeric Hymn to Demeter, King Celeus is said to have been one of the first people to learn the secret rites and mysteries of her cult. He was also one of her original priests, along with Diocles, Eumolpos, Polyxeinus and Triptolemus, Celeus’ son, who had supposedly learned agriculture from Demeter. 
Under Peisistratos of Athens, the Eleusinian Mysteries became pan-Hellenic, and pilgrims flocked from Greece and beyond to participate. Around 300 BC, the state took over control of the Mysteries; they were controlled by two families, the Eumolpidae and the Kerykes. This led to a vast increase in the number of initiates. The only requirements for membership were freedom from “blood guilt”  , meaning never having committed murder, and not being a “barbarian” (being unable to speak Greek). Men, women and even slaves were allowed initiation. 
To participate in these mysteries one had to swear a vow of secrecy.
Four categories of people participated in the Eleusinian Mysteries:
1. Priests, priestesses, and hierophants.
2. Initiates, undergoing the ceremony for the first time.
3. Others who had already participated at least once. They were eligible for the fourth category.
4. Those who had attained épopteia (Greek: ποπτεία) (English: “contemplation”), who had learned the secrets of the greatest mysteries of Demeter.
The outline below is only a capsule summary; much of the concrete information about the Eleusinian Mysteries was never written down. For example, only initiates knew what the kiste, a sacred chest, and the kalathos, a lidded basket, contained.
Hippolytus of Rome, one of the Church Fathers writing in the early 3rd century CE, discloses in Refutation of All Heresies that “the Athenians, while initiating people into the Eleusinian rites, likewise display to those who are being admitted to the highest grade at these mysteries, the mighty, and marvellous, and most perfect secret suitable for one initiated into the highest mystic truths: an ear of grain in silence reaped.” 
There were two Eleusinian Mysteries, the Greater and the Lesser. According to Thomas Taylor, “the dramatic shows of the Lesser Mysteries occultly signified the miseries of the soul while in subjection to the body, so those of the Greater obscurely intimated, by mystic and splendid visions, the felicity of the soul both here and hereafter, when purified from the defilements of a material nature and constantly elevated to the realities of intellectual [spiritual] vision.” According to Plato, “the ultimate design of the Mysteries … was to lead us back to the principles from which we descended, … a perfect enjoyment of intellectual [spiritual] good.” 
The Lesser Mysteries took place in the month of Anthesteria under the direction of Athens’ archon basileus. In order to qualify for initiation, participants would sacrifice a piglet to Demeter and Persephone, and then ritually purify themselves in the river Illisos. Upon completion of the Lesser Mysteries, participants were deemed mystai (“initiates”) worthy of witnessing the Greater Mysteries.
The first act (14th Boedromion) of the Greater Mysteries was the bringing of the sacred objects from Eleusis to the Eleusinion, a temple at the base of the Acropolis of Athens.
The Greater Mysteries took place in Boedromion, the third month of the Attic calendar, falling in late summer, and lasted ten days. On 15th Boedromion, called Agyrmos “the Gathering”, the hierophants (priests or “those who show the sacred ones”) declared prorrhesis, the start of the rites, and carried out the “Hither the victims” sacrifice (hiereía deúro). The “Seawards initiates” (halade mystai) began in Athens on 16th Boedromion with the celebrants washing themselves in the sea at Phaleron.
On 17th Boedromion, the participants began the Epidauria, a festival for Asklepios named after his main sanctuary at Epidauros. This “festival within a festival” celebrated the hero’s arrival at Athens with his daughter Hygieia, and consisted of a procession leading to the Eleusinion, during which the mystai apparently stayed at home, a great sacrifice, and an all-night feast (pannykhís). 
The procession to Eleusis began at Kerameikos (the Athenian cemetery) on 19th Boedromion from where the people walked to Eleusis, along what was called the “Sacred Way” ( ερ δός, Hierá Hodós), swinging branches called bacchoi. At a certain spot along the way, they shouted obscenities in commemoration of Iambe (or Baubo), an old woman who, by cracking dirty jokes, had made Demeter smile as she mourned the loss of her daughter. The procession also shouted “Íakch’, O Íakche!” referring to Iacchus, possibly an epithet for Dionysus, or a separate deity, son of Persephone or Demeter. 
Upon reaching Eleusis, there was an all-night vigil (pannychis) according to Mylonas  and Kerenyi.  perhaps commemorating Demeter’s search for Persephone. At some point, initiates had to down a special drink of barley and pennyroyal, called kykeon, which has led to speculation about its chemicals perhaps having psychotropic effects.
Then on 20th and 21st Boedromion, the initiates entered a great hall called Telesterion; in the center stood the Anaktoron (“palace”), which only the hierophants could enter, where sacred objects were stored. Before mystai could enter the Telesterion, they would recite, “I have fasted, I have drunk the kykeon, I have taken from the kiste (“box”) and after working it have put it back in the kalathos (“open basket”). 
It is widely supposed that the rites inside the Telesterion comprised three elements: dromena (“things done”), a dramatic reenactment of the Demeter/Persephone myth; deiknumena (“things shown”), displayed sacred objects, in which the hierophant played an essential role; and finally legomena (“things said”), commentaries that accompanied the deiknumena. 
Combined these three elements were known as the aporrheta (“unrepeatables”); the penalty for divulging them was death. Athenagoras of Athens, Cicero, and other ancient writers cite that it was for this crime (among others) that Diagoras was condemned to death in Athens; 
the tragic playwright Aeschylus was allegedly tried for revealing secrets of the Mysteries in some of his plays, but was acquitted.  The ban on divulging the core ritual of the Mysteries was thus absolute, which is probably why we know almost nothing about what transpired there.
As to the climax of the Mysteries, there are two modern theories. Some hold that the priests were the ones to reveal the visions of the holy night, consisting of a fire that represented the possibility of life after death, and various sacred objects. Others hold this explanation to be insufficient to account for the power and longevity of the Mysteries, and that the experiences must have been internal and mediated by a powerful psychoactive ingredient contained in the kykeon drink. (See “entheogenic theories” below.)
Following this section of the Mysteries was the Pannychis, an all-night feast 
accompanied by dancing and merriment. The dances took place in the Rharian Field, rumored to be the first spot where grain grew. A bull sacrifice also took place late that night or early the next morning. That day (22nd Boedromion), the initiates honored the dead by pouring libations from special vessels.
On 23rd Boedromion, the Mysteries ended and everyone returned home
In 170 AD, the Temple of Demeter was sacked by the Sarmatians but was rebuilt by Marcus Aurelius. Aurelius was then allowed to become the only lay person ever to enter the anaktoron. As Christianity gained in popularity in the 4th and 5th centuries, Eleusis’s prestige began to fade. The last pagan emperor of Rome, Julian, reigned from 361 to 363 after about fifty years of Christian rule. Julian attempted to restore the Eleusinian Mysteries and was the last emperor to be initiated into them. 
The Roman emperor Theodosius I closed the sanctuaries by decree about 30 years later, in 392 AD. The last remnants of the Mysteries were wiped out in 396 AD, when Arian Christians under Alaric, King of the Goths, destroyed and desecrated the old sacred sites.  The closing of the Eleusinian Mysteries in the 4th century is reported by Eunapius, a historian and biographer of the Greek philosophers. Eunapius had been initiated by the last legitimate Hierophant, who had been commissioned by the emperor Julian to restore the Mysteries, which had by then fallen into decay. According to Eunapius, the very last Hierophant was a usurper, “the man from Thespiae who held the rank of Father in the mysteries of Mithras.”
Despite the destruction of the Eleusinian Mysteries, elements of the cult survived in the Greek countryside. There, Demeter’s rites and religious duties were partially transferred by peasants and shepherds onto Saint Demetrius of Thessaloniki, who gradually became the local patron of agriculture and “heir” to the pagan mother goddess.