The Romans took this feast originally from the Greeks, who called it ΑΝΑΒΑΣΙΣ, q.d. Ascensus: the eve of that day they spent in tears and lamentations, and denominated it ΚΑΤΑΒΑΣΙΣ, Descensus. Afterwards, the Greeks took the name ΙΛΑΡΙΑ, from the Romans, as appears from Photius's Bibliotheca, in his codex of the life of the philosopher Isidore of Alexandria.
The term seems originally to have been a name which was given to any day or season of rejoicing. The hilaria were, therefore, according to Maximus Monachus either private or public. Among the former, he thinks it the day on which a person married, and on which a son was born; among the latter, those days of public rejoicings appointed by a new emperor. Such days were devoted to general rejoicings and public sacrifices, and no one was allowed to show any symptoms of grief or sorrow.
But the Romans also celebrated hilaria, as a feria stativa, on the eighth day before the Kalends of April—March 25—in honour of Cybele, the mother of the gods; and it is probably to distinguish these hilaria from those mentioned above, that the Augustan History  calls them Hilaria Matris Deûm. The day of its celebration was the first after the vernal equinox, or the first day of the year which was longer than the night. The winter with its gloom had died, and the first day of a better season was spent in rejoicings. The manner of its celebration during the time of the republic is unknown, except that Valerius Maximus mentions games in honour of the mother of the gods. Respecting its celebration at the time of the empire, we learn from Herodian that, among other things, there was a solemn procession, in which the statue of the goddess was carried, and before this statue were carried the most costly specimens of plate and works of art belonging either to wealthy Romans or to the emperors themselves. All kinds of games and amusements were allowed on this day; masquerades were the most prominent among them, and everyone might, in his disguise, imitate whomsoever he liked, and even magistrates.
Festival structure 
The hilaria were in reality only the last day of a festival of Cybele, which commenced on March 22, and was solemnised by the Galli with various mysterious rites. It must, however, be observed that the hilaria are neither mentioned in the Roman calendar nor in Ovid's Fasti.
The full festival can be tentatively reconstructed (with the days of the festival literally translated) as follows:
- 15 March. 'The Reed Entered'. Its exact significance is uncertain (the reeds may refer to the river bank where Attis was exposed as a child and rescued by Cybele.) A nine-day period of abstinence from bread, pomegranates, quinces, pork, fish and probably wine began. Only milk was permitted as a drink.
- 22 March. 'The Tree Entered' (Arbor intrat). A pine tree from a wood sacred to Cybele is felled following the sacrifice of a ram at its roots. The tree was carried in procession through the city as if in a funeral to the Temple of Cybele on the Palatine Hill.
- 23 March. A day of mourning.
- 24 March. 'The Day of Blood' (Sanguis). Frenzied rites including scourging and whipping. Castration rituals would take place on this day. the tree is symbolically buried.
- 25 March. 'The Day of Joy' (Hilaria) celebrating the resurrection of Attis. This was the hilaria proper (as opposed to the mournful tone of the previous days).
- 26 March. A day of rest.
- 27 March. 'The Washing' (Lavatio). Added by Marcus Aurelius.
- 28 March. Possible ceremony at the Vatican sanctuary. Appears in the Calendar of Philocalus.
Herodian details an assassination plot by Maternus against Emperor Commodus that was to occur on the hilaria. Maternus planned to disguise himself and his followers as members of the Praetorian Guard, and proceed among the true members of the Guard, until they were close enough to kill Commodus. However, one of Maternus's followers revealed the plot ahead of time, betraying him because, according to Herodian, his men "preferred a legitimate emperor to a robber tyrant." On the day of hilaria, he was beheaded and his followers punished. The public celebrated the emperor's safety, and Commodus sacrificed to Cybele for protecting him from harm.
According to the Calendar of Philocalus from 354, a Hilaria of Isis was celebrated on November 3.
- This article incorporates text from a publication now in the public domain: Smith, William, ed. (1870). "article name needed". Dictionary of Greek and Roman Antiquities. London: John Murray.
- This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "article name needed". Cyclopaedia, or an Universal Dictionary of Arts and Sciences (first ed.). James and John Knapton, et al.
- Mary Beard, John North, Simon Price. Religions of Rome. Cambridge University Press. 1998. ISBN 0-521-45646-0. pp 133–134.
- Robin Osborne. Studies in Ancient Greek and Roman Society. Cambridge University Press. 2004. ISBN 0-521-83769-3. p 365.
- "Hilaria". Oxford English Dictionary. Oxford University Press. 2nd ed. 1989.