Festivals are periods of time, cut out from daily life, during which a group performs activities that are most often thought of as communications with the superhuman world. Festival names in Greece and Rome often express this close connection with a divinity, a hero, or a human founder, or they refer to a ritual activity that is characteristic for a festival. The basic ritual elements that underlie a specific festival scenario are similar in both cultures (as well as in other cultures of the ancient Mediterranean world): processions, sacrifices with ensuing banquets, and athletic and musical contests are most common and exist already in the festival descriptions in Homer, such as the New Moon festival on Ithaca in the Odyssey. Common festivals founded and expressed group identity, first and foremost on the city level, but also for smaller and larger groups, from the family and clan group to the tribe or the community of all Hellenes. Greek and Roman festivals were so similar in their basic forms that, during the Imperial epoch, cities in the eastern part of the Empire adopted Roman festivals despite the fact that Greek cities followed a lunar calendar, whereas Rome early on had introduced a luni-solar system. The one festival type absent from the Roman world, at least during the Republican epoch, was the mystery ritual that, typically through a one-time initiation ritual, founded groups that transcended a single city, as well as the limits of gender and social status.
During the Imperial epoch, both Rome and the cities of Greece continued their traditional festivals, but also developed their festival calendars in new directions, continuing and exploring innovations that had occurred already in Hellenistic times. An early development was ruler cult, developed in the Greek cities during Hellenistic times and adopted for the cult of Roman emperors, who exploited its potential to tie together a heterogeneous empire through shared cultic activities. The most important driving force was an understanding of divine power that was defined through its helpful manifestation and thus allowed the cult of outstandingly powerful humans. Wealthy citizens of Hellenistic cities also founded festivals in the memory of family members, and during the Imperial period, such foundations multiplied and gained in grandeur. The Imperial epoch also saw the extension of single festivals to events that lasted many days, if not an entire month and helped to shape the Christian festival calendar with its long festival periods.