The saeculum was a measurement of time used by the Etruscans and Romans to represent a single lifetime: no-one who witnessed the beginning of a saeculum would see its end, by definition.
Obsolete measurements of time are a small obsession of mine (wait until I write about Medieval atoms and momenta!). The saeculum is a neat concept: a period of time roughly equivalent to the maximum human lifetime, and therefore equivalent to the renewal of the entire human race.
Take an important date, like the founding of Rome. According to Roman myth, the city of Rome was founded around 750 BCE. After about 110 years, everyone who was alive at the time of Rome’s founding would have died – no-one living would remember that significant event. That period of time, 110 years, is one saeculum.
Romans used this concept mainly as an excuse to have a party. Good ol’ Romans, eh? The Secular Games were a big event that occurred once every century or so, an occasion to sacrifice animals to the gods, go to the theatre, and race chariots. There were actually two separate sets of Secular Games, one lining up with Emperor Augustus’ games of 17 BCE and another begun by Emperor Claudius in 47 CE to celebrate the 800th anniversary of the founding of Rome.
These two Secular Games alternated for the next few hundred years: Domitian and Septimus Severus held them one and two saecula (respectively) after Augustus and Antoninus Pius and Philip I one and two saecula after Claudius. Then Christianity hit the Roman Empire and such pagan festivals were dropped.
(In case you were wondering, this is where we get the word secular from – although it did not pick up its connotation of “worldly” in contrast to the eternity of the divine until much later.)