AGONALIA, or AGONIA
One of the most ancient festivals at Rome, celebrated several times in the year. Its institution, like that of other religious rites and ceremonies, was attributed to Numa Pompilius. We learn from the ancient calendars that it was celebrated on the three following days, the 9th of January, the 21st of May, and the llth of Becember (A. D. V. Id. Jan.; XII. Kal. Jun. III. Id. Dec.); to which we should probably add the 17th of March (A. D. XVI. Kal. Apr.), the day on which the Liberalia was celebrated, since this festival is also called Agonia or Agonium Martiale. The object of this festival was a disputed point among the ancients themselves; but as Hartung has observed, when it is recollected that the victim which was offered was a ram that the person who offered it was the rex sacrificulus, and that the place where it was offered was the regia, we shall not have much difficulty in understanding the significance of this festival. The ram was the usual victim presented to the guardian gods of the state, and the rex sacrificulus and the regia could be employed only for such ceremonies as were connected with the highest gods and affected the weal of the whole state. Regarding the sacrifice in this light, we see a reason for its being offered several times in the year.
The etymology of the name was also a subject of much dispute among the ancients; and the various etymologies that were proposed are given at length by Ovid. None of these, however, are at all satisfactory; and we would therefore suggest another. It is well known that the Quirinal hill was originally called Agomis, and the Colline gate Agonensis. What is then more likely than that this sacrifice should have been originally offered on this hill, and should thence have received the name of Agonalia? It is expressly stated that the sacrifice was offered in the regia, or the domus regis, which in the historical times was situated at the top of the sacra via, near the arch of Titus; but in the earliest times, the regia is stated by an ancient writer to have been upon the Quirinal, and this statement seems to render our supposition almost certain.
The Circus Agonensis, as it is called, is supposed by many modern writers to have occupied the place of the present Piazza Navona, and to have been built by the emperor Alexander Severus on the spot where the victims were sacrificed at the Agonalia. Becker has however, brought forward good reasons for questioning whether this was a circus at all, and has shown that there is no authority whatever for giving it the name of circus Agonensis.
Dictionary of Greek and Roman Antiquities by William Smith