A saeculum, in Roman religious reckoning, was the longest possible span of a human life, the length of time until every person alive at a given moment had died, and Roman antiquarian tradition held that the beginning of each new saeculum was marked by prodigies that only priestly experts, consulting the Sibylline Books, could correctly identify and act on. The resulting festival, the Ludi Saeculares or Secular Games, was consequently rare almost by definition, meant to occur only once in what Roman writers reckoned as roughly a hundred years, so that in principle no one alive could expect to see the rite performed twice, a fact Roman sources present as central to its solemnity.
The clearest ancient description of the festival's religious program comes from the Republican and early Imperial periods and centers on a three-day, three-night structure: nights given to chthonic sacrifice at the buried altar of Dis Pater and Proserpina on the Campus Martius, days turning outward to sacrifices and games honoring Jupiter, Juno and other state gods associated with Rome's continued prosperity, so that the festival moved deliberately from confronting mortality and the underworld toward affirming the city's renewed future.