Poplifugia -- a festival of origins which were forgotten by the time folks began writing about things; it possibly commemorates the flight of the people from Gauls in the fourth century, but that seems a rather strange thing for Romans to build a festival around (even with the story of Tutula/Philotis* attached to it).
*From Seyffert's Dictionary of Classical Antiquities, s.v. Caprotina:
The festival [sc. of Juno Caprotina] was connected with another, called Poplifugium, or the "Flight of the People," held on the 5th of July. Thus a historical basis was given to it, though the true origin of both festivals had been probably forgotten. After their defeat by the Gauls, the Romans were conquered and put to flight by a sudden attack of their neighbours, the Latins, who demanded the surrender of a large number of girls and widows. Thereupon, at the suggestion of a girl called Tutula (or Philotis), the female slaves disguised themselves as Roman ladies, went into the enemy's camp, and contrived to make the enemy drunk, while Tutula, climbing a wild fig-tree, gave the signal for the Romans to attack by holding up a torch. The Poplifugia were celebrated by a mimic flight. [...]
Here's the account from Plutarch's Life of Camillus:
They say that the Latins (whether out of pretence, or real design to revive the ancient relationship of the two nations) sent to desire of the Romans some free-born maidens in marriage; that when the Romans were at a loss how to determine (for on one hand they dreaded a war, having scarcely yet settled and recovered themselves, and on the other side suspected that this asking of wives was, in plain terms, nothing else but a demand for hostages, though covered over with the specious name of intermarriage and alliance), a certain handmaid, by name Tutula, or, as some call her, Philotis, persuaded the magistrates to send with her some of the most youthful and best-looking maid-servants, in the bridal dress of noble virgins, and leave the rest to her care and management; that the magistrates, consenting, chose out as many as she thought necessary for her purpose, and adorning them with gold and rich clothes, delivered them to the Latins, who were encamped not far from the city; that at night the rest stole away the enemy's swords, but Tutula or Philotis, getting to the top of a wild fig-tree, and spreading out a thick woollen cloth behind her, held out a torch towards Rome, which was the signal concerted between her and the commanders, without the knowledge, however, of any other of the citizens, which was the reason that their issuing out from the city was tumultuous, the officers pushing their men on, and they calling upon one another's names, and scarce able to bring themselves into order; that setting upon the enemy's works, who either were asleep or expected no such matter, they took the camp and destroyed most of them; and that this was done on the Nones of July, which was then called Quintilis, and that the feast that is observed on that day is a commemoration of what was then done. For in it, first, they run out of the city in great crowds, and call out aloud several familiar and common names, Caius, Marcus, Lucius, and the like in representation of the way in which they called to one another when they went out in such haste.
[I'm experimenting here]