“mundus patet”, the gate to the underworld on the Forum Romanum was opened.

24 August: One of the three days of the year was celebrated in Rome, when “mundus patet”, the gate to the underworld on the forum was opened.
"When the mundus is open, it is as if a door stands open for the sorrowful gods of the underworld." (Macrobius, “Saturnalia“)

Alice Pike Barney's somewhat ghostly imagination of Ceres (1901)

Plutarch hands down the story of Romulus, who, after he slew and buried his brother Remus, dug a pit in the low ground between the Palatine, Capitoline and Esquiline Hills, were one day the Forum Romanum would be, and the first fruits were deposited in it as a gift to Ceres, the goddess of agriculture who once descended into Hades to bring back her daughter Proserpina, a story about Nature’s eternal death and rebirth. Death entered the scenery pretty soon and Romulus’ pit became a gateway where the spirits of deceased ancestors, the manes, could enter this world. To keep this access limited and every part of the family tree where it belonged, the pit, shaped like the inverse dome of the sky, was covered with a stone, the lapis manalis, and the abode became the Mundus Cerialis, the world of Ceres.

A gate to the underworld on the Forum Romanum

To appease the guardians of the Underworld as well as the agricultural deities, the offerings of the first fruits were still made immediately after the Consualia, when grain was ritually stored in subterranean vaults. Then, on three different days of the year in autumn, the lapis manalis was removed. Now was one of the occasions when the di manes, the sacred spirits of the ancestors, could leave the nether world and roam the city, and even though the day was probably not considered a religious holiday, it was not thought wise to transact business, marry or declare a war. When the day ended, the lapis manalis, the ghost stone, would again be placed on the mundus and the spirits would return to where they belonged and the holiday for the ghosts was over.

The Navel of the World - Rome's Umbilicus Urbis

Opinions differ whether the mundus was actually located on the forum and some place it on the Palatine Hill. Usually it is associated with the umbilicus urbis on the forum where a small temple curves like a navel over the underbelly of the city. The location, right beside the Arch of Severus, had indeed the reputation to be a place where the under and the upper world touch. From here, all distances in the Empire were once measured and posted, carved in the marble structure of the small edifice, 350 miles to Carthage, 900 to Londinium or 1,200 to Alexandria. What remains today of the old navel of the world is a small, forlorn looking brick structure, with the marble all gone.

Yours truly narrating the tale with more imagery - click on "Play"