A religious festival is a time of special importance marked by adherents to that religion. Religious festivals are commonly celebrated on recurring cycles in a calendar year or lunar calendar. The science of religious rites and festivals is known as heortology. Roman festivals Festivals (feriae) were an important part of Roman religious life during both the Republican and Imperial eras, and were one of the primary features of the Roman calendar. Feriae ("holidays" in the sense of "holy days") were either public (publicae) or private (privatae). State holidays were celebrated by the Roman people and received public funding. Feriae privatae were holidays celebrated in honor of private individuals or by families. The 1st-century BC scholar Varro defined feriae as "days instituted for the sake of the gods." A deity's festival often marked the anniversary (dies natalis, "birthday") of the founding of the deity's temple, or a rededication after a major renovation. Public business was suspended for the performance of religious rites on the feriae. Cicero says that people who were free should not engage in lawsuits and quarrels, and slaves should get a break from their labors. On calendars of the Republic and early Empire, the religious statusdays were marked by letters such as F (for fastus), and N (for nefastus, when political activities and the administration of justice were prohibited). By the late 2nd century AD, extant calendars no longer show these letters, probably as a result of calendar reforms undertaken by Marcus Aurelius that recognized the changed religious environment of the empire. On surviving Roman calendars, festivals that appear in large capital letters (such as the Lupercalia and Parilia) are thought to have been the most ancient holidays, becoming part of the calendar before 509 BC. Some of the oldest festivals are not named for deities. During the Imperial period, several traditional festivals localized at Rome became less important, and the birthdays and anniversaries of the emperor and his family gained prominence as Roman holidays.