It's not only Friday the 13th, it's also August 13th: a day devoted to Hecate, although this may be a modern, rather than ancient, tradition.
There is, however, widespread belief among modern worshippers that [Hecate] has a feast day on August 13 to protect the crops from violent storms.
Wikipedia is perpetuating this belief, citing Leo Ruickbie's "Witchcraft out of the Shadows" (2004). In a side-box he claims that the ancient Greeks observed a feast day on August 13 in which Hecate was propitiated to not send storms to destroy the growing crops. Ruikbie, in turn, cites his source as Diane Stein's "The Goddess Book of Days" (Llewellyn, 1997). Her original calendar was published in 1988 and does not give a primary source.
Various Internet sites claim that this occurred in the House of Storms and Fertility, that it was a Festival to Hecate of the Moon, or that it was part of the Festival of Hecate and Artemis. Mikalson, in The Sacred and Civil Calendar of the Athenian Year lists for Metageitnion 16 that "the sacrifical calendar of the deme Erkhia prescribes sacrifices on this day to Kourotrophos and Artemis Hekate.". Metageitnion is the Attic lunar month that lines up with late July/August. . . . Unfortunately, I cannot find what occurred during the rite (if anything specific at all). Still, this doesn't explain why August 13 was chosen, fixed as it is to the Roman solar year instead of the lunar calendar used by the ancient Greeks.
Also, there is never any mention as to why Hecate would be called to protect crops. According to Brumfield in his book The Attic festivals of Demeter and their Relation to the Agricultural Year (1976), during the time of the year we call August, the grain harvest had been completed and the grape harvest would not have begun until September. August was a lull in the agricultural year and nothing needed to be protected 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.
None of this explains a connection with storms or harvests, however. This strikes me as a purely Neopagan phenomenon rising out of widespread observance of harvest-type rituals during early August, the most common being the Celtic feast of Lughnasadh. From a theological point of view, perhaps this dark-dressed flashing eyed goddess calling herself Hecate has inspired us to set Her feast day. We are creating new religions, after all.
It's certainly been stormy here in the MidAtlantic the last few days and I'm as happy to celebrate a new tradition as an old one.
For me, the association between Hecate and summer storms makes some sense. We're at a liminal time. Summer's almost past and Autumn's not quite started. Storms bring chaos, change, that liminal unpredictability that can knock over old trees in an instant, flood a familiar roadway, turn an almost ripe crop into mush. And, so, tonight, I will light incense for Hecate, lay out some of her traditional foods, and chant for the crops.
Picture found here.