According to "A Dictionary of Greek and Roman Antiquities" (1890) (eds. William Smith, LLD, William Wayte, G. E. Marindin) 
- ...four festivals in the Roman calendar. Many different explanations have been given of the name (cf. Ov. Fast. i. 319-332; Fest. Epit. s. v.; Varr. L. L. vi. 12), most of them quite absurd. There can be little doubt that it is derived from the Old Latin agonia, a victim, or agonium, a festival, a word also found in other Italian dialects (Corssen, i.2 396; Curtius, Principles, i. 209).
- 1. On Jan. 9 the rex offered in the regia a ram in honour of Janus (Cal. Praenest.; Ov. Fast. i. 333). This sacrifice was said to have been instituted by Numa (Macrob. i. 4, 7).
- 2. On March 17, the day of the Liberalia, called according to Masurius (apud Macrob. i. 4, 15) the Agonium Martiale, a victim was offered by the Salii agonales on the Mons Quirinalis, hence sometimes called Mons Agonus, in honour of Mars, or more probably of Quirinus. (Cal. Vatic.)
- 3. On May 21 there was a festival called in the Cal. Venus. Agon. Vediovi, when a victim was offered to Vejovis in his temple on the Capitoline hill.
- 4. On Dec. 11 there was, according to Festus, a festival called Septimontium, when sacrifice was offered on each of the seven hills. This is denoted in the Fasti Amiternini (C. I. L., p. 325) AG. IN., interpreted by Mommsen as Agonia Inui. (Cf. Marquardt, Röm. Staatsv. iii. p. 310, note; Huschke, Das alte röm. Jahr, Breslau, 1869, pp. 247-8.)