The lustrum was a ceremonial sacrifice performed in ancient Rome. This was performed by one of the censors (a Roman magistrate) every five years after the census was taken. New senators were elected to office after the ceremony. After this rite, the magistrates would pledge to the gods that the people would repeat the offering in five years if they protected the people.
The rite itself was simple. The people would gather on a field dedicated to Mars in his aspect of fertility god. A sheep, a pig and a bull were chosen and led in a circuit around the people by individuals with 'blessed' names like Felix (lucky) or Dives (happy). After this circuit the animals were sacrificed to Mars and completely burned.
The lustrum was intended to cleanse the slate for the people, and it's use of people with blessed names to ward them against evil. Similar ceremonies can be found in Persia and Macedonia though Roman observance could be spotty; use of the lustrum lapsed for a period of 41 years until Octavian decreed it's restoration.
Such purification rites can be inserted into any story where populations with connections to the divine occur (few don't). Characters may have to officiate at such ceremonies (census taking offers it's own risks & rewards) or take part. The consequences of such a ceremony may offer respite from a curse or disease or even offer a very minor protective boon.
Those games featuring politics may use the lustrum as a method of purging the populace of any crimes it may have committed; purifying the people and their elected officials of any evil they may have brought upon themselves in doing state business. Such a performance may merely salve some consciences, others may have more practical or mystical consequences.