The Equirria (also as Ecurria, from *equicurria, "horse races") were two ancient Roman festivals of chariot racing, or perhaps horseback racing, held in honor of the god Mars, one February 27 and the other March 14.
The Equirria took place in the Campus Martius, the "Field of Mars," outside the sacred boundary of Rome (pomerium). The exact course is debated: perhaps near the Altar of Mars in the campus; or on the Tarentum, the site of the ludi tarentini, which became the Saecular Games; or the Trigarium. When the Tiber flooded, the Equirria were transferred to the Campus Martialis on the Caelian Hill, a field without permanent structures.
On the calendar 
The Equirria were said to have been founded by Romulus, the son of Mars. Both appear on the oldest Roman calendars inscribed on stone. The Equirria are part of what Michael Lipka calls "temporal focalization" in the Roman conception of deity. The festivals of Mars—the February 27 Equirria, a feria on the Kalends of March (a day sacred also to his mother Juno), Agonalia March 17, Tubilustrium March 23, the ritual of the October Horse October 15, and Armilustrium October 19—cluster at his namesake month (Latin Martius), except for festivals of Mars in October to close the military campaigning season.
In the earliest form of the calendar, the year began with March, and thus the February 27 Equirria originally preceded New Year's Day, and was the last festival for Mars of the year. The March 14 Equirria occurs the day before the Ides, when the Roman people celebrated the feast of Anna Perenna, whose name expresses her role as a goddess of the year (Latin annus; cf. English "perennial"). The March 14 Equirria and the Regifugium ("King's Flight") are the only such festivals to fall on an even-numbered date. Despite scholarly efforts, no explanation for this displacement has found wide acceptance. Georg Wissowa thought the March Equirria had originally occurred on the Ides, and was moved up a day because of conflicts among religious events concentrated around this ritually fraught time; an alternate view is that it was placed "at some convenient day" between the Nones and the Ides. At any rate, the horse races framed the ritual turn of the year, and the difficulties of the placement of the two Equirria arise from changes made to the calendar, when January became the first month.
Originally, the Equirria may have featured races on horseback, like the archaic festivals of the Consualia and Taurian Games, rather than chariot races. The gods of the underworld (di inferi) were characteristically propitiated by horse racing in the Campus Martius with "old and obscure" festivals such as the Consualia, at sites including the Tarentum and the Trigarium. Hendrik Wagenvoort speculated that the archaic Mars "had been imagined as the god of death and the underworld in the shape of a horse."
William Warde Fowler understood the Equirria as "lustrations of the horse" for the army. They occur during what most scholars see as a general "war festival" for Mars. The end of the campaigning season was marked in October, with the ritual of the October Horse, which also involved chariot races, on the Ides, and the Armilustrium on October 19. The paucity of evidence on the Equirria, as with other archaic festivals, may indicate that they were preserved for the sake of religious tradition, but not attended by masses of people.
- John H. Humphrey, Roman Circuses: Arenas for Chariot Racing (University of California Press, 1986), p. 560.
- Ovid, Fasti 2.860; Jörg Rüpke, The Roman Calendar from Numa to Constantine: Time, History, and the Fasti, translated by David M.B. Richardson (Wiley-Blackwell, 2011), p. 75.
- Hendrik Wagenvoort, "The Origin of the Ludi Saeculares," in Studies in Roman Literature, Culture, and Religion (Brill, 1956), p. 224.
- Humphrey, Roman Circuses, pp. 560 and 684–685 (note 72). Robert E.A. Palmer, Studies of the Northern Campus Martius in Ancient Rome (American Philosophical Society, 1990), pp. 28–29, doubts that the Trigarium was ever anything but a practice field.
- Festus 117 in the edition of Lindsay; Ovid, Fasti 2.519–520; Gregory S. Aldrete, Floods of the Tiber in Ancient Rome, pp. 38, 67, 79, 96; Robert E.A. Palmer, The Archaic Community of the Romans (Cambridge University Press, 2009), p. 150; Humphrey, Roman Circuses, p. 560.
- Geoffrey S. Sumi, Ceremony and Power: Performing Politics in Rome between Republic and Empire (University of Michigan Press, 2005), p. 288, with additional sources.
- Wagenvoort, "The Origin of the Ludi Saeculares," p. 225.
- Michael Lipka, Roman Gods: A Conceptual Approach (Brill, 2009), p. 37. The views of Georg Wissowa on the festivals of Mars framing the military campaigning season are summarized by C. Bennett Pascal, "October Horse," Harvard Studies in Classical Philology 85 (1981), p. 264, with bibliography.
- Wagenvoort, "The Origin of the Ludi Saeculares," pp. 224–227; Frank Bernstein, "Complex Rituals: Games and Processions in Republican Rome," in A Companion to Roman Religion (Blackwell, 2007), p. 223; C. Bennett Pascal, "October Horse," Harvard Studies in Classical Philology 85 (1981), p. 6; Robert Turcan, The Gods of Ancient Rome (Routledge, 2001), p. 65; Lipka, Roman Gods, p. 39; William Warde Fowler, The Roman Festivals of the Period of the Republic (London, 1908), p. 44ff. Fowler, The Religious Experience of the Roman People (London, 1922), p. 99, notes that about the pecular even-date placement of the Regifugium and March Equirria "we know so little that it is almost useless to speculate as to the reason for their exception from the rule." See also Bernstein, "Die römischen Ecurria / Equirria — kriegerische Feste?" Nikephoros 12 (1999) 149–169. See discussion throughout Rüpke, The Roman Calendar.
- Rüpke, Roman Calendar, p. 76.
- Tertullian, De spectaculis 5, says that Romulus had replaced the Consualia with the Equirria.
- Wagenvoort, "The Origin of the Ludi Saeculares," p. 228.
- Humphrey, Roman Circuses, pp. 544, 558; Auguste Bouché-Leclercq, Manuel des Institutions Romaines (Hachette, 1886), p. 549; "Purificazione," in Thesaurus Cultus et Rituum Antiquorum (LIMC, 2004), p. 83. See also the Lusus Troiae.
- Wagenvoort, "The Origin of the Ludi Saeculares," p. 228.
- Fowler, Religious Experience, pp. 98, 217.
- Hendrik Wagenvoort, "On the Magical Significance of the Tail," in Pietas: Selected Studies in Roman Religion (Brill, 1980), p. 148; John Scheid, An Introduction to Roman Religion (Indiana University Press, 2003), p. 51.
- Fowler, Roman Festivals, p. 249 et passim.
- Jörg Rüpke, "Communicating with the Gods," in A Companion to the Roman Republic (Wiley-Blackwell, 2010), p. 224.