Poplifugia or Populifugia was a Roman festival celebrated on July 5th. Translated from Latin, the name of the festival is: “the day of the people’s flight”.
It appears that the ancient Romans did not even know the meaning behind this festival, but there were several theories. It seems someone attacked Rome and the people fled to safety, but a few days later, Rome rallied and subdued their enemies. Who the attacker was, is lost to history. Varro believed it to have been the Gauls, Macrobius the Tuscans and Plutarch says the Latins, although Plutarch also suggests that this festival actually commemorates the death of Romulus rather than any military action and the flight of the people was their rush out of Rome to get to the place where he disappeared to make sacrifices.
The event, whatever it was, is linked to Nonae Caprotinae.
The festival itself involved people being chased out of the /regia (King’s home) and the sacrifice of a bull and the reading of its entrails.
Poplifugia and Festival for Romulus in Rome at the Prot-Indo-European Religion Home Page
The Poplifugia: a Roman Festival Celebrating a Flight at decodedpast.com