The romans held special ceremonies, the ludi saeculares (the games of the century) every 100-110 year. The original concept was that a new saeculum begins when the last person born at the beginning of the previous one died.

Later they fixed it to 110 years (but sometimes it was just 100 year. yes, I know it's a bit messy). Another problem is that the date of the celebration was based on the date of the foundation of Rome. The known datation of -753 came from Varro in the middle of the first century BC. In addition August changed a bit the date of the games to announce a new golden age of peace after the civil war in the games of -17. There were games in -249 and -149 so there should have been games in -49 but they were not organized because of the civil war. Because of the games of August two dates of the saeculi competed and were used for political reasons by various emperors using the date of Varro or continuing the computation of August. That's why there were 6 games between -17 and 247.

So in 247 a new "century" was proclaimed (less than 50 years after the previous one) Luckily, it was in the middle of the reign of Philip the arab, a period of relative stability in the middle of the 3rd century, just before the great period of anarchy of the second half of that century.