Rites in honour of Juno Sospita: Juno Sospita was originally worshipped in Lanuvium, where she seems to have had started out as a fertility goddess of some sort and evolved into a warrior protectrix of the city. When Lanuvium was granted Roman citizenship in 338 B.C., the cult was also given special status and place under the control of the pontifices, who would annually perform a sacrifice to her. There also seems to have been a ritual whereby blindfolded girls would enter her grove to feed barley cakes to the sacred snakes therein. If the cakes were accepted, the girls were proven to be virgins and the fertility for the upcoming year was guaranteed. Which of these rituals -- or perhaps both -- took place on this day isn't clear in my sources.
Rites in honour of Elernus: Elernus (or Helernus, or maybe Avernus) is another one of those very ancient Roman deities about which we know little, as can be seen by the variations in name. He appears to have been some type of underworld divinity (perhaps being honoured with the sacrifice of a black ox by the pontifices).
1793 - death of John Lempriere (Classical Dictionary)