The ancient Greek calendars were luni-solar. The years were marked by the solstices or equinoxes (region dependent), for example the first day of the year in Athens was the first new waxing moon after the summer solstice. The months thereafter were purely lunar, beginning on the first waxing moon, ending on the dark moon. This means that no matter how we spin it, the calendars back then can never match our calendar today. There was no August 13th, because there was no August. And even if there was an August, it would not be the same time as it is now, but instead would be based on the solstices and lunar months.
August generally coincides with the month of Metageitnion in the Attic calendar, this year Metageitnion began on July 25th and will end on August 22nd. On the 16th day of Metageitnion, this year August the 9th, the festival of Kourotrophos was (and is) held. Kourotrophos is the name of the Goddess who protects children and is also an epithet of numerous Goddesses, including Artemis and Hekate. Thus, for this particular festival Hekate and Artemis, as Kourotrophos, were and are honoured, offered to and sacrificed to.
Though this festival has nothing in common with the modern 13th festival, we can see something else that could match up. The date. Since the calendars don’t match up, the 16th of Metageitnion is on a different day in August (perhaps rarely July and/or September) each year. This means it isn’t impossible for the Kourotrophos festival to land on August 13th, in fact it is highly likely to have happened.
Whether this could have contributed to the date of Hekates Night is impossible to know. But fact is it wouldn’t be the first time a festival is given a static annual date based on a confusion over a yearly changeable date. I suspect this is what happened with one (or both) of the November festivals to Hekate, but with the Deipnon instead of an annual festival.
If we look back to the years before our first modern link was published in 1988, we can see that the full moon was on August 11th in 1984, and two days later would have been the Kourotrophos festival – the 13th.
