Today is a very packed holy-day, and unfortunately I won't have time to elaborate upon it as much as I'd prefer.
First and foremost, it's the dies natalis of Diana, as celebrated throughout Italy, and particularly at her shrine at Lake Nemi, which was near the community of Aricia and was strongly identified with it; but, it was also close to Lanuvium, where Antinous and Diana shared a temple and acollegium, and thus it makes it into their festival calendar as a major holy-day. I'll have more to say on this in a moment. In the meantime, you can listen to my Natalis Dianae hymn from my entry on this day two years ago.
It's also a festival of the Dioskouroi in Rome, as well as being a festival of Vertumnus and of Hercules Invictus (the second day of a two-day festival for him, actually). As Antinous is syncretized to all of these deities/heroes, we thus also reckon his syncretism festivals for them on this day. You can read slightly more about these occasions from last year's entry.Via http://aediculaantinoi.wordpress.com/2012/08/13/natalis-dianae-and-virbius/