This festival is one celebrating Neptune, from Classical Living: Reconnecting with the Rituals of Ancient Rome: Myths, Gods, Goddesses, Celebrations, and Rites for Every Month of the Year by Frances Bernstein, Ph.D. It takes place in July, but I thought I’d share it. Of course, it’s Poseidon we are studying, and the question of whether the Greek and Roman Gods are the same Deities becomes relevant here. In general, I treat many of Them, the “Big Gods” (i.e. the Olympians, in contrast to the smaller, more local Gods, especially the agricultural ones. Faunus, for example, I don’t treat as being the same Deity as Pan.), as the same or similar until I have a reason not to. I may throw in a phrase here or there such as “if You be the same Deity”, or “By whichever name You wish to be known”, in acknowledgment of the fact that I don’t know for sure Who They are. I can’t, not really, because I’m mortal and as far as some of this goes its just theorizing, trying to make sense of my experiences to understand Beings far greater than I.
If I do get it wrong, I don’t think the Gods mind, since it’s made out of love and genuine effort. And if I am worshiping Neptune instead of Poseidon, there’s nothing wrong with that. All the Gods deserve veneration. In any case, here’s the information. YMMV. Make of it what you will.
“July 23, X Kalends August
Neptunus was the ancient Italic God of Water — most likely in the oldest time of freshwater or springwater. Neptunus protected all waterways and human activates linked to water. His festival was held in the hottest month, July. His counterpart was Salacia, Goddess of Leaping Water or Spring Water. Neptunus was also connected with Venilia, the Roman Goddess of Coastal Water, although neither Salacia nor Venilia had rituals of their own. On July 23, shady arbors were made of leaves for the worshipers. which undoubtedly helped protected them from the sun. An altar to Neptunus in Circus Flaminus was dedicated on this day in third century B.C.E. The offering and prayers to Neptunus were for a continued supply of water during the the hottest days of the years. ….
Water is not only restorative, but essential in the heat of July Since the oldest of times, water has symbolized birth, healing, and renewal.” (Page 140-141)