Wikipedia
The Amburbium ("City Circuit", from ambire, "to go around" + urb-, "city"; plural amburbia) was an ancient Roman festival for purifying the city; that is, a lustration (lustratio urbis). It took the form of a procession, perhaps along the old Servian Wall, though the length of 10 kilometers would seem impractical to circumambulate. If it was a distinct festival held annually, the most likely month is February, but no date is recorded and the ritual may have been performed as a "crisis rite" when needed.
The Amburbium can be hard to distinguish from the Ambarvalia in ancient sources, either because it was a similar set of ritual procedures performed on behalf of the city instead of the fields or rural areas (arva), or because both originated with the priesthood of the Arvales, "Brothers of the Fields". Vopiscus sees the two as closely related: "the city is purified, the hymns are chanted, the Amburbium is celebrated, the Ambarvalia is carried out." Both festivals seem to have involved the sacrifice of a pig, a sheep, and a bull ( suovetaurilia). The Amburbium's sacrificial victims ( hostiae) were amburbiales. According to Servius, for the Ambarvalia a hostia with the capacity to produce felicitas ("fecundity, blessedness") is led around in a ritual circuit three times; the ceremony, he says, is called an amburbium when it is the city that is circumambulated. The encircling (circuire) is identical with the purification (lustrare).
Amburbium does not appear on any of the ancient calendars, and is thus assumed if annual to be one of the feriae conceptivae, a moveable feast. Macrobius, an antiquarian writer of late antiquity, says that the semi-legendary second king of Rome Numa added Ianuarius and Februarius to the end of the ten-month calendar of Romulus, and instituted a lustration of the city in February, with the sacrifices to be offered to the Di Manes. The Amburbium is not named as such in the passage, but H.H. Scullard thought it might be meant. Most festival activity in February pertained to the care and propitiation of the dead. The scarcity of evidence may indicate that in the Imperial period the Amburbium was celebrated irregularly as needed, but it was performed as late as 271 AD. According to the Historia Augusta, on January 11 of that year the emperor Aurelian ordered a consultation of the Sibylline books, a collection of prophetic utterances from the gods ( fata deorum), resulting in a lustration of the city by means of the Amburbium and Ambarvalia.
The ritual has been compared to the lustral sacrifices described in the Iguvine Tablets, which were conducted by the Fratres Atiedii, a "brotherhood" of priests at Iguvium (present-day Gubbio). It is one of several ceremonies of ancient Roman religion in which a sacred topography is marked out through a procession.