Of all the mysteries celebrated in ancient times, the mysteries at Eleusis are assumed to be of great importance to a large portion of the ancient Hellenes. The cult itself likely has origins dating back to the Mycenean period of around 1600 to 1100 BC and it is believed that the cult of Demeter Herself was established in 1500 BC. 

The Eleusinian mysteries (Ἐλευσίνια Μυστήρια) had festivals throughout the year, which were tied to agriculture through Demeter's refusal to perform her duties as an agricultural Theia while her daughter Persephone is with Hades, and to the afterlife and Underworld through Persephone's return to the surface of the earth after Her mandatory stay with Hades has ended. Initiation ceremonies were held every year at Eleusis. The Mysteries were mysteries for a reason and revealing information of these involved rituals carried the punishment of death (or at least, that is what the ancient sources say). We still have been able to piece together quite a bit of information however, and I would like to create a masterpost of sorts.

Mythological connections and sources of the Mysteries:
The Eleusinian Mysteries 
A short introduction to the Mysteries, which includes the mythological reason for the festival and the rewards for being initiated.
On Persephone and Spring 
A writer's perspective on the myth of Persephone's abduction. 
The personification of the ritual cry of joy called ‘iakhe’ of the procession of the initiates. 
On 'Kore' versus 'Persephone' 
...and why They are not always the same.

Eleusinian festivals:
A summary of the festivals included in the cycle of the Mysteries. It also includes a description of the way the days of the Mysteries would have been spent--as well as we can gather about a mystery tradition, of course. The cycle consists of: the Greater Mysteries (13-23 Boedromion), Proerosia (6 Pyanepsion), Stenia (9 Pyanepsion), Thesmophoria (11-13 Pyanepsion), Haloa (26 Poseideon), the Lesser Mysteries (20-26 Anthesterion), and the Skiraphoria (12 Skirophorion). These are placed in sequence of the Athenian year. Also linked to the Mysteries because they fall during either the Lesser of Greater Mysteries: the Diasia and the Epidauria.

Periferal points of interest connected to the Mysteries:
On attending women-only festivals as a male
Some of the eleusinian rites are women-only. Can you attend as a male?
On attending women-only festivals as non-binary
Some of the eleusinian rites are women-only. Can you attend if you identify as non-binary?
On maidens, the Stenia and the Thesmophoria 
On attending women-only festivals as an unmarried woman.
On the binding of hair 
As part of Eleusinian ritual.
Dadoukhoi: torch bearers 
On the importance of torches and torch bearers in the Eleusinian Mysteries.
The ancient tradition of Martes 
Martes were pieces of string, worn around the wrist. The innitiates of the Mysteries recieved yellow ones on the way to Eleusis.
On ending the Mysteries
On why the ending rites of the Mysteries matters.
On fasting for the Eleusinian Mysteries
Fasting is a part of the Greater Mysteries as well as the Thesmophoria. Here is how to do it safely.
On the forbidden foods of the Eleusinian Mysteries 
Some things not to eat during these days.
On the 'hiera' of the Eleusinian Mysteries
The heira are the 'sacred objects'.
On kykeon 
A barley beverage sacred to Demeter and used to break a sacred fast within the Eleusinian Mysteries as well as in preparatory rites for some of the most sacred--and secret--rites within Eleusis. 
Pelanoi, the cakes of the Athenians 
Athenian sacrifical cakes that were often used in the Lesser mysteries.
Snakes, the ancient Hellenes and the Eleusinian Mysteries 
Snakes were used in the Lesser Mystery rites. Why?