A History of the Feast of Hekate
To read the entire article, refer to the original source: https://www.otherworld-apothecary.com/blog/2015/08/the-origins-of-the-feast-of-hecate/
(Edited for length)
There’s not much in the classical literature about [Hecate] being associated with storms, beyond that Zeus ‘gave’ her power over all realms. Her dominion here instead seems to grow out of the “dark and stormy night” image that she developed during the middle ages. There is, however, widespread belief among modern worshipers that she has a feast day on August 13 to protect the crops from violent storms.

A few clues come to light when we stop looking for ancient Greek sources. In Rome, The Festival of Torches was held on August 13, called the Nemoralia. In it, woman would walk from the city of Rome carrying torches to a lake sacred to Diana where they would offer their petitions. There was a strong conflation between Artemis and Hecate in Greece, with Hecate taking on a number of Artemis’ roles. Diana and Hecate were also conflated some, but typically maintained separate spheres of influence. Still, this seems to be a likely source for fixing the ritual on that particular date.
Additionally, in 1986 a ritual performed on August 14, 1985, was published in Circle Network News which invoked Hecate Chthonia and incorporated a Hecate Supper. A web page by that author claims that a similar ritual incorporating much of the same text was performed at the MoonStone Circle of the Aquarian Tabernacle and published in Panegyria on August 13, 1988. The original date it was performed, August 14, 1985, was a dark moon, which has been a sacred time for Hecate since classical times. The other date, though, perhaps inspired by Stein’s recently published Goddess Book of Days, was a waxing gibbous.

We still suspect the modern Feast of Hecate held on August 13 comes from the Nemoralia, the festival of Diana held in the groves at Nemi. This cult has a long association with modern paganism, being the inspiration and central study in Sir James Frazer’s The Golden Bough which, in turn, formed a pillar of the neo-pagan movement.
The goddess Diana as she was worshiped in the groves as Nemi possessed a triple form, not unlike the triform figure of Hekate that is familiar to many modern witches. One of the three was known classically as Hecate or Proserpina, something which has troubled me. Why is a Latin Goddess being called by the name of a different Greek goddess? Is it syncretism, like the conflation of Artemis and Hecate, and Artemis and Diana. CMC Green in Roman Religion and the Cult of Diana at Aricia offers this plausible explanation: “The identification of Diana with Hecate (a Greek name) has been made unnecessarily complicated. Diana the Huntress was identified with the moon, as Apollo was with the sun. As the moon grows dark once a month it is inevitable that a moon-goddess will have some part of her identity located in the underworld. Hecate is simply the Greek name for that part of her identity.” The names Hecate and Proserpina were also likely considered safe substitutes for the true name of the Underworld Moon.

There are numerous classical references to this association. One of Horace’s Odes mentions Diva Triformis, and Virgils Dido calls on “tergeminanque hecaten, tria virginis ora Dianae.” Isodore of Seville writing in the first century explains: “Concerning which Virgil writes..the three faces of the virgin Diana, because the same goddess is called Luna, Diana, and Proserpina”. This tripartate Diana persisted through the centuries, showing up in triple form in Chaucer’s Canterbury Tales and John Skelton’s Garland of Laurels in 1523 (“Diana in the leaves green, Luna that so bright doth sheen, Persephone in hell”). As mentioned previously, her cult instigated James Frazer’s life work The Golden Bough and influenced Robert Graves’ The White Goddess, through which the concept of the triple goddess was introduced to modern Wicca.

To return to the August festival which honored the three-fold goddess, Green’s translation of one of the poems composed by the Latin poet Statius in the 1st century CE is appropriate:
It is the season when the most scorching region of the heavens takes over the land and the keen dog-star Sirius, so often struck by Hyperion’s sun, burns the gasping fields. Now is the day when Trivia’s Arician grove, convenient for fugitive kings, grows smoky, and the lake, having guilty knowledge of Hyppolytus, glitters with the reflection of the multitude of torches; Diana herself garlands the deserving hunting dogs and polishes the arrowheads and allows the wild animals to go in safety, and at virtuous hearths all Italy celebrates the Hecatean Ides.
Finally, Green suggests that the festival lasted 3 days, starting with her descent to the underworld on the Ides (August 13th) where she would be known as Hecate, and culminating on the 15th of August when she ascended as the Queen of Heaven, the full moon. Incidentally the 15th is celebrated as the Feast of the Assumption of Mary, Queen of Heaven, in the Christian church, who may have adopted this (non-infernal) part of the festival.
Today, the August 13th Feast of Hecate has grown immensely in popularity among modern pagans, and includes many individual observances as well as larger public rites such as Hecate’s Feast hosted by the Temple of Witchcraft. We conclude that the Hecate honored at these rites isn’t necessarily the Greek goddess of boundaries or Lady of Storms, but they are an indirect continuation of rites to the dread face of Diva Triformis as goddess of night, the queen of the dark moon roaming the underworld. From the classical torch-lit rites within the grove at Nemi to modern observances by pagans and witches, August 13th brings together all those who form the conclave of the goddess of night. May she bless you all.