In ancient Roman religion, the Parilia is a festival of rural character performed annually on April 21, aimed at cleansing both sheep and shepherd. It is carried out in acknowledgment to the Roman deity Pales, a deity of uncertain gender who was a patron of shepherds and sheep.
Ovid describes the Parilia at length in the Fasti, an elegiac poem on the Roman religious calendar, and implies that it predates the founding of Rome, traditionally 753 BC, as indicated by its pastoral, pre-agricultural concerns. During the Republic, farming was idealized and central to Roman identity, so the festival took on a more generally rural character. Increasing urbanization caused the rustic Parilia to be reinterpreted rather than abandoned, as Rome was an intensely traditional society. During the Imperial period, the date was celebrated as the "birthday" of Rome (dies natalis Romae).
The pastoral structure of the festival is carried out by the shepherd himself. After the sheep pen had been decorated with green branches and a wreath draped on the gate, the remainder of the ceremony took place in sequence. At the first sign of daylight, the shepherd would purify the sheep: by sweeping the pen and then constructing a bonfire of straw, olive branches, laurel, and sulfur. The noises produced by this burning combination were interpreted as a beneficial omen. The shepherd would jump through this flame, dragging his sheep along with him. Offerings of millet, cakes, and milk were then presented before Pales, marking the second segment of the ceremony. After these offerings, the shepherd would wet his hands with dew, face the east, and repeat a prayer four times. Such prayers requested Pales’ assistance in freeing the shepherd and the flock from evils brought about by accidental wrongdoings (e.g. trespassing on sacred grounds and removing water from a sacred water source). The final portion of the rural festival made use of the beverage burranica, a combination of milk and sapa (boiled wine). After consumption of this beverage, the shepherd would leap through the fire three times, bringing an end to the ceremony.
The urban form of the Parilia, on the other hand, is blended with other Roman religious practices and carried out by a priest. Ovid personally participated in this form and describes his experiences in the Fasti. While the central actions of the rural ceremony carry over, the urban form adds two ingredients from other religious festivals: the Fordicidia and the October Horse. The Fordicidia sacrifices a pregnant cow to the deity Tellus to promote cattle and field fertility. The unborn calf is then removed from the womb and burnt. The October Horse is the right hand horse of the team that won a particular chariot race on October 15 of the previous year. Together, the ashes of the unborn calf and the blood from the head of the October Horse are mixed by the Vestals and are added to the burning bean straw of the bonfire. Dumézil questioned whether the Equus October provided the horse blood, since the two ancient sources that mention the ingredient omit identifying the victim.
By the end of the late Republic, the Parilia became associated with the birthday of Rome. Numerous accounts of the founding of Rome exist, but the particular one related to the Parilia is described by Ovid in the Fasti. According to this myth, Romulus, upon reaching Rome on the day of the Parilia, took a stick and engraved a line in the ground that defined the boundaries of the new city (pomerium). He then prayed to the gods Jupiter, Mars, and Vesta asking for protection of this area. However, his brother Remus, unaware of the boundaries, crossed the line and was struck down by Romulus's henchman Celer.
Over time, and under the influence of several Roman rulers, the structure of the Parilia changed. First, after Julius Caesar heard the news of Roman Victory at Munda in 45 BC (around the date of the Parilia), he added games to the ceremony. At these games, the citizens would wear crowns in Caesar’s honor. Caligula instituted into the celebration a procession of priests, noblemen, boys and girls of noble birth singing of his virtues while escorting the Golden Shield, previously bestowed upon him by the citizens of Rome, to the Capitol. At this time the Parilia became Rome's birthday celebration rather than the rural festival it had once been. In 121 AD Hadrian founded a new temple of Venus and Roma and changed the festival’s name to Romaea. The temple was ruined. 
- The Cambridge Ancient History 2nd Ed. Vol. X: The Augustan Empire 43 BC – AD 69. Cambridge University Press. Great Britain: 1996. pp. 816-817
- Fowler, Warde W. The Roman Festivals of the Period of the Republic. MacMillan and CO, Limited. London: 1899. pp. 79-85.
- Butrica, James L. Propertius on the Parilia (4.4.73-8). Classical Quarterly 50.2. Memorial University of Newfoundland. Great Britain: 2000. pp. 472-478.
- Ovid. Fasti; Gower, John. Ovids Festivalls, or Romane calendar, translated into English verse equinumerally. London: 1640. pp. 93-97.
- Adkins, Lesley & Roy A. Dictionary of Roman Religion. Facts on File Inc. New York: 1996. pp. 82, 168.
- G. Dumezil La religion romaine archaique Paris, 1974, part 1, chap. 4 "Mars".
- Propertius Elegiae Romanae 4, 1, 19-20 (where the horse is described as curtus, "dismembered"); Ovid Fasti IV, 721 sqq. The October Horse, however, is the only horse sacrifice known to have been practiced regularly by the Romans.
- Kearns, Emily. The Oxford Dictionary of Classical Myth and Religion. Oxford University Press. Oxford: 2003. pp. 406.
- Charles River Editors (2013-02-14). Caligula & Nero: Rome’s Worst Emperors (Kindle Locations 715-716). Charles River Editors. Kindle Edition.